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{{short description|Banded variety of chalcedony}}
{{Short description|Banded variety of chalcedony}}
{{other uses}}
{{Other uses}}
{{Good article}}
{{Infobox mineral
{{Infobox mineral
| name = Agate
| name = Agate
| category = [[Tectosilicate]] [[minerals]], [[quartz]] [[Mineral group|group]], [[chalcedony]] [[Mineral variety|variety]]
| category = [[Tectosilicate]] minerals  
| group = [[Quartz]] group
| IMAstatus = [[Mineral variety|Variety]] of quartz ([[chalcedony]])
| boxwidth =
| boxwidth =
| boxbgcolor =#bb361c
| boxbgcolor =#bb361c
| boxtextcolor = #fff
| boxtextcolor = #fff
| image = Mexican Crazy Lace Agate - World's Best.jpg
| image = Malawi Agate (Malawi, southeastern Africa) (32734668126).jpg
| imagesize = 260px
| imagesize =  
| caption = {{cvt|19.6|kg|lb|0}} specimen of crazy lace agate from Chihuahua, Mexico next to a tennis ball; {{cvt|38.2|cm|in}} wide
| caption = Polished agate nodule from [[Malawi]]
| formula = SiO<sub>2</sub> ([[silicon dioxide]])
| formula = SiO<sub>2</sub> ([[silicon dioxide]])
| molweight =
| molweight =
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}}
}}


'''Agate''' ({{IPAc-en|ˈ|æ|ɡ|ɪ|t}} {{respell|AG|it}}) is a banded variety of [[chalcedony]]. Agate stones are characterized by alternating bands of different colored chalcedony and sometimes include [[Macroscopic scale|macroscopic]] quartz. They are common in nature and can be found globally in a large number of different varieties. There are some varieties of chalcedony without bands that are commonly called agate ([[moss agate]], [[fire agate]], etc.); however, these are more properly classified solely as varieties of chalcedony. Agates are primarily formed as nodules within [[volcanic rock]], but they can also form in [[Vein (geology)|veins]] or in [[sedimentary rock]]. Agate has been popular as a [[gemstone]] in [[Jewellery|jewelry]] for thousands of years, and today it is also popular as a collector's stone. Some duller agates sold commercially are artificially dyed to enhance their color.
'''Agate''' ({{IPAc-en|ˈ|æ|ɡ|ɪ|t}} {{respell|AG|it}}) is a banded variety of fibrous [[chalcedony]]. Agate stones are characterized by alternating bands of different colored chalcedony and often contain some visible [[quartz]] crystals. They are common in nature and can be found globally in a large number of different varieties. Most agates are concentrically banded, while some have bands that form parallel lines. Certain varieties of chalcedony without bands are commonly called agate (e.g. [[moss agate]], [[fire agate]]), but these are not true agates. Moreover, not every banded chalcedony is an agate (e.g. banded [[chert]]).  


==Etymology ==
Most agates form as nodules within [[volcanic rock]], either as [[amygdule]]s or [[thunder egg]]s. Agates also form in [[Vein (geology)|veins]] within any rock type and in [[Silicification|silicified]] fossils. The processes that result in agate banding are not well understood; agate has never been made synthetically.  
Agate was given its name by [[Theophrastus]], a [[Greeks|Greek]] [[philosopher]] and [[naturalist]]. He discovered the stone c. 350 BCE along the shoreline of the River Achates ({{langx|grc|Ἀχάτης}}), now the [[Dirillo|Dirillo River]], on the [[Italy|Italian]] island of [[Sicily]], which at the time was a Greek territory.<ref name="pabian" />{{rp|pp=52,162}}


==Formation and properties==
Agate has been popular as a [[gemstone]] in [[Jewellery|jewelry]] for thousands of years, and today it is also popular as a collector's stone. Many duller agates sold commercially are artificially treated to enhance their color.
[[File:Agate- & quartz-lined geode 5 (32375570960).jpg|thumb|Geode agate]]


Agates are most commonly found as [[Nodule (geology)|nodules]] within the cavities of [[volcanic rock]]s<ref name="Moxon" /> such as [[basalt]], [[andesite]], and [[rhyolite]]. These cavities, called [[Vesicle (geology)|''vesicles'']] (''amygdaloids'' when filled),<ref name="pabian" />{{rp|p=12}} are gas bubbles that were trapped inside the lava when it cooled.<ref name="Moxon">{{Cite journal |last=Moxon |first=T |last2=Reed |first2=S. J. B. |year=2006 |title=Agate and chalcedony from igneous and sedimentary hosts aged from 13 to 3480 Ma: a cathodoluminescence study |url=https://pubs.geoscienceworld.org/gsa/minmag/article/70/5/485/140307 |url-status=live |journal=Mineralogical Magazine |volume=70 |issue=5 |pages=485–498 |bibcode=2006MinM...70..485M |doi=10.1180/0026461067050347 |s2cid=54607138 |archive-url=https://web.archive.org/web/20220313041353/https://pubs.geoscienceworld.org/gsa/minmag/article/70/5/485/140307 |archive-date=March 13, 2022 |access-date=October 1, 2006|url-access=subscription }}</ref><ref name="lynch formation">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=11–13}}</ref><ref name="pabian" />{{rp|p=12}} The vesicles are later filled with hot, silica-rich water from the surrounding environment, forming a silica gel. This gel crystallizes through a complex process to form agates. Since agates usually form in lavas poor in free silica, there are multiple theories of where the silica originates from, including micro-shards of silica glass from [[volcanic ash]] or [[tuff]] deposits and decomposing plant or animal matter.<ref name="pabian" />{{rp|p=11}} Agates are much harder than the rocks they form in, so they are frequently found detached from their host rock.
==Etymology==
The earliest known description of agate was {{circa|350 [[BCE]]}} by [[Theophrastus]], a Greek [[philosopher]] and [[naturalist]].{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=52,162}} According to both Theophrastus and the Roman naturalist [[Pliny the Elder]],<ref name="OED" /> it was named for its discovery along the [[Dirillo|River Achates]] ({{langx|grc|Ἀχάτης}}) in [[Sicily]].{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=52,162}}<ref>{{cite book |last1=Theophrastus|last2=Caley |first2=Earle R. |last3=Richards |first3=John F.C |title=Theophrastus. On Stones |date=1956 |publisher=The Ohio State University |location=Columbus, Ohio |page=52 |quote=The achates is also a beautiful stone; it comes from the river Achates in Sicily and is sold at a high price. |url=https://archive.org/details/theophrastus-stones-caley-richards-1956/page/52/mode/2up |access-date=6 May 2026}}</ref><ref>{{cite book |last1=Pliny the Elder |last2=Bostock |first2=John |last3=Riley |first3=H. T. |title=Naturalis historia |publisher=Perseus Digital Library |url=http://www.perseus.tufts.edu/hopper/text?doc=Plin.+Nat.+37.54 |access-date=6 May 2026 |chapter=Book 37, Chapter 54 |quote=Achates was a stone formerly in high esteem, but now held in none. It was first found in Sicily, near a river of that name; but has since been discovered in numerous other localities.}}</ref> However, etymologists believe the river was probably named after the stone,<ref name="OED" /><ref name="MW" /> and the origin of the name is uncertain.<ref name="OED">[https://www.oed.com/dictionary/achate_n1?tab=etymology#30020355 "Achate, N. (1), Etymology."] ''Oxford English Dictionary''. Oxford University Press. Retrieved 6 May 2026. {{doi|10.1093/OED/6418139009}}</ref> The word ''agate'' first appeared in English in the 15th century, derived from the [[Anglo-Norman language|Anglo-Norman French]] ''agathe'' and the [[Latin]] ''achātēs''.<ref name="MW">{{cite web |title=Agate |url=https://www.merriam-webster.com/dictionary/agate#word-history |website=merriam-webster.com |publisher=Merriam-Webster |access-date=6 May 2026}}</ref>


Geologists generally understand the early stages of agate formation, but the specific processes that result in band development are still widely debated. Since they form in cavities within host rock, agate formation cannot be directly observed,<ref name="lynch formation" /> and unlike most other crystals, agates have never been successfully lab-grown.<ref>{{cite web |last1=Brown |first1=Nancy Marie |title=How Do Agates Form? |url=https://www.psu.edu/news/research/story/how-do-agates-form |website=psu.edu |publisher=The Pennsylvania State University |date=31 August 2001|access-date=3 March 2025}}</ref>
==Composition==
Agate is composed principally of chalcedony,<ref name="mindat" /> a [[Microscopic scale|microscopic]] ([[microcrystalline]]) or submicroscopic ([[cryptocrystalline]]) form of [[silica]]. Most of the silica in chalcedony is quartz, while less than 5% to 20% is [[moganite]], a quartz [[Crystal polymorphism|polymorph]].<ref name="chalcedony" /><ref name="Heaney 1992">{{cite journal |last1=Heaney |first1=Peter J. |last2=Post |first2=Jeffrey E. |title=The Widespread Distribution of a Novel Silica Polymorph in Microcrystalline Quartz Varieties |journal=Science |date=24 January 1992 |volume=255 |issue=5043 |page=442 |doi=10.1126/science.255.5043.441 |pmid=17842895 |bibcode=1992Sci...255..441H |url=https://www.science.org/doi/10.1126/science.255.5043.441 |access-date=26 April 2026}}</ref> All forms of silica, including quartz and moganite, have a chemical composition of {{chem2|SiO2}}. They differ in that quartz has [[trigonal]] symmetry while moganite is [[monoclinic]].<ref name="chalcedony" /><ref name="moganite">{{cite web |title=Mogánite |url=https://www.mindat.org/min-2739.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=26 April 2026}}</ref> Over time, the less-stable moganite is converted to quartz;<ref name="chalcedony" />{{sfn|Götze|Möckel|Pan|2020|loc=4.1.1}}<ref name="Moxon 2004" /> as a result, no moganite has been found in agates dating from before the [[Silurian]] period.<ref name="Moxon 2004" />  


Agate is composed of multiple bands, or layers, of chalcedony fibers,<ref>{{cite journal |last1=Wang|first1=Yifeng |last2=Merino|first2=Enrique |date=1990-06-01 |title=Self-organizational origin of agates: Banding, fiber twisting, composition, and dynamic crystallization model |journal=[[Geochimica et Cosmochimica Acta]] |language=en |volume=54 |issue=6 |pages=1627–1638 |doi=10.1016/0016-7037(90)90396-3 |bibcode=1990GeCoA..54.1627W |issn=0016-7037}}</ref> specifically length-fast chalcedony fibers and sometimes quartzine (length-slow chalcedony fibers).<ref name="mindat" /> Agate can also contain [[opal]], an [[amorphous]], hydrated form of silica.<ref name="pabian" />{{rp|p=11}} In ''wall-banded agates'', the fibers grow radially from the vesicle walls inward, perpendicular to the direction of the bands.<ref name="mindat">{{Cite web |title=Agate |url=https://www.mindat.org/min-51.html |access-date=10 February 2025 |website=mindat.org |publisher=Hudson Institute of Mineralogy}}</ref><ref>{{Cite web|url=https://www.ingentaconnect.com/content/schweiz/njma/2009/00000186/00000002/art00001|title=The formation of agate structures: models for silica transport, agate layer accretion, and for flow patterns and flow regimes in infiltration channels|last1=Walger|first1=Eckart|last2=Mattheß|first2=Georg|date=August 2009|website=www.ingentaconnect.com|language=en|access-date=March 3, 2020|last3=von Seckendorff|first3=Volker|last4=Liebau|first4=Friedrich|archive-date=June 4, 2018|archive-url=https://web.archive.org/web/20180604062121/https://www.ingentaconnect.com/content/schweiz/njma/2009/00000186/00000002/art00001|url-status=live}}</ref> The vesicle walls are often coated with thin layers of [[celadonite]] or [[chlorite]],<ref name="lynch formation"/><ref name="pabian" />{{rp|p=18}} soft, green [[phyllosilicate]] minerals that form from the reaction of hot, silica-rich water with the rock.<ref name="lynch formation" /> This coating provides a rough surface for the chalcedony fibers to form on, initially as radial [[spherulites]]. The rough surface also causes agate husks to have a pitted appearance once the coating has been weathered away or removed.<ref name="pabian" />{{rp|pp=18–19}} Sometimes, the spherulites grow around mineral inclusions, resulting in eyes, tubes, and sagenitic agates. The first layer of spherulitic chalcedony is typically clear, followed by successive growth bands of chalcedony alternated with chemically precipitated color bands, primarily [[iron oxides]].<ref name="pabian" />{{rp|p=13}} The center is often macrocrystalline quartz (quartz with visible crystals),<ref name="lynch formation" /> which can also occur in bands and forms when there is not enough water in the silica gel to promote chalcedony [[polymerization]].<ref name="pabian" />{{rp|p=18}} When the silica concentration of the gel is too low, a hollow center forms, called an agate ''geode''.<ref name=":02">{{Cite web|url=http://www.minerals.net/MineralDefinition.aspx?name=agate|title=Agate chalcedony: The mineral Agate information and pictures|website=www.minerals.net|language=en-US|access-date=2020-02-27|archive-date=2020-03-16|archive-url=https://web.archive.org/web/20200316005135/https://www.minerals.net/MineralDefinition.aspx?name=agate|url-status=live}}</ref><ref name="pabian" />{{rp|p=17}}
Unlike [[Macroscopic scale|macroscopic]] (macrocrystalline) quartz, which is [[anhydrous]], the chalcedony in agate normally contains small amounts of water,<ref name="chalcedony" />{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=11}} between 0.5% and 3%. Most is chemically bound in the form of [[silanol]] ([[Silicon|Si]]–[[Hydroxy group|OH]]), with lesser amounts of molecular {{chem2|H2O}}.{{sfn|Götze|Möckel|Pan|2020|loc=4.2.3}} Silanol presence decreases with age before levelling off at about 0.4% as a side effect of the conversion of moganite to quartz; this is a potentially useful indicator of age in younger agates.{{sfn|Götze|Möckel|Pan|2020|loc=4.2.3}}<ref name="Moxon 2004">{{cite journal |last1=Moxon |first1=Terry |last2=Ríos |first2=Susana |title=Moganite and water content as a function of age in agate: an XRD and thermogravimetric study |journal=European Journal of Mineralogy |date=29 March 2004 |volume=16 |issue=2 |pages=269–278 |doi=10.1127/0935-1221/2004/0016-0269 |bibcode=2004EJMin..16..269M |url=https://www.schweizerbart.de/papers/ejm/detail/16/55886/Moganite_and_water_content_as_a_function_of_age_in?af=crossref |access-date=26 April 2026}}</ref> Macrocrystalline quartz is also present in most agates, particularly in the center as a solid core or [[geode]],{{sfn|Lynch|Lynch|2012|pp=11-13}}{{sfn|Götze|Möckel|Pan|2020|loc=4.1.1}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=18}} but also occasionally in bands.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=17}}
Quartz forms crystals around the cavity, with the apex of each crystal pointing towards the center. Occasionally, the quartz may be colored, such as [[amethyst]] or [[smoky quartz]]. ''Level-banded agates'' form when chalcedony precipitates out of solution in the direction of gravity, resulting in horizontal layers of microscopic chalcedony spherulites.<ref name="mindat" /> [[Enhydro agate|''Enhydro agates'']], or ''enhydros'', form when water becomes trapped within an agate (or chalcedony) nodule or geode, often long after its formation.<ref>{{cite book |last1=Bates |first1=R. L. |last2=Jackson |first2=J. A. |title=Glossary of Geological Terms |date=1987 |publisher=American Geological Institute |location=Alexandria, Virginia |page=788 |edition=3rd |url=https://www.gamineral.org/writings/enhydros-gray.html |access-date=9 March 2025}}</ref><ref>{{cite web |title=Enhydro Agate |url=https://www.mindat.org/min-7596.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=9 March 2025}}</ref>


Agates can also form within rock fissures, called ''veins''.<ref name="pabian" />{{rp|pp=11–12}} Vein agates form in a manner similar to nodular agates, and they include most lace agates, such as blue lace agate and crazy lace agate.
Agate may also contain small amounts of [[opal]], an [[amorphous]] form of silica{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=11}} containing up to 10% molecular water.{{sfn|Götze|Möckel|Pan|2020|loc=4.2.3}} Most is in the form of opal-CT, which consists of [[Nanocrystalline material|nanometer-sized crystals]] of the silica polymorphs [[cristobalite]] and [[tridymite]] and is therefore not truly amorphous. Fully amorphous opal-A is rare in agates. Like moganite, both varieties of opal are [[metastable]] and are eventually converted to quartz.{{sfn|Götze|Möckel|Pan|2020|loc=4.1.1}}


Less commonly, agates can form as nodules within [[sedimentary rock]], such as [[limestone]], [[Dolomite (rock)|dolomite]] or [[tuff]]. These agates form when silica replaces another mineral, or silica-rich water fills cavities left by decomposed plant or animal matter.<ref name="pabian" />{{rp|p=11–12}} Sedimentary agates also include ''fossil agates'', which form when silica replaces the original composition of an organic material.<ref>{{cite web |title=Fossil Agate |url=https://www.mindat.org/min-7603.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=9 March 2025}}</ref> This process is called [[silicification]], a form of [[petrification]]. Examples include [[petrified wood]],<ref>{{cite web |title=Petrified Wood |url=https://www.mindat.org/min-8018.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=9 March 2025}}</ref> agatized coral,<ref>{{cite web |title=Agatized coral |url=https://www.mindat.org/min-43510.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=9 March 2025}}</ref> and Turritella agate ([[Elimia tenera]]).<ref name="turritella" /> Although these fossils are often referred to as being "agatized", they are only true agates when they are banded.<ref name="mindat" />
===Impurities===
Agate often contains [[Inclusion (mineral)|mineral inclusions]] or [[Trace element|trace elements]] incorporated into the crystal structure. The most common trace elements present in agate are [[aluminum]], [[calcium]], [[potassium]], [[sodium]], and [[iron]].{{sfn|Götze|Möckel|Pan|2020|loc=4.2.1}} Iron is responsible for producing the color of most agate bands;{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=21}} red and yellow bands are often colored by microscopic spheres of the iron oxides [[hematite]] and [[goethite]], respectively.{{sfn|Götze|Möckel|Pan|2020|loc=4.2.1}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=21}} [[Manganese]], [[chromium]], and [[nickel]] may also be present in some colored bands.{{sfn|Götze|Möckel|Pan|2020|loc=4.2.1}} Some agates contain very small traces of [[uranium]] in the form of [[uranyl]] ions, causing them to fluoresce green under short-wave [[ultraviolet]] light.<ref>{{cite journal |last1=Götze |first1=Jens |last2=Gaft |first2=Michael |last3=Möckel |first3=Robert |title=Uranium and uranyl luminescence in agate/chalcedony |journal=Mineralogical Magazine |date=August 2015 |volume=79 |issue=4 |pages=985-995 |doi=10.1180/minmag.2015.079.4.08 |url=https://www.cambridge.org/core/journals/mineralogical-magazine/article/abs/uranium-and-uranyl-luminescence-in-agatechalcedony/3B1A829796AEEE72494F4603CCB0B6CA |access-date=4 May 2026}}</ref><ref name="JAES" /> The most common mineral inclusions in agate are [[calcite]] and [[zeolite]]s.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=17,19}}


==Varieties (by structure)==
==Structure==
Agates are broadly separated into two categories based the type of banding they exhibit.<ref name="lynch water-level" /><ref name="pabian" />{{rp|pp=24,36}} ''Wall banding'', also called ''concentric banding'' or ''adhesional banding'', occurs when agate bands follow the shape of the cavity they formed in. ''Level banding'', also called ''water-level banding,'' ''gravitational banding'', ''horizontal banding'', ''parallel banding'', or ''Uruguay-type banding'', occurs when agate bands form in straight, parallel lines. Level banding is less common and usually occurs together with wall banding.<ref name="mindat" />
===Fibers===
[[File:Agate Rio Grande do Sul Caillois Donation MNHN n18.jpg|thumb|When backlit, thin slices of agate can occasionally show their fibrous structure as lines running perpendicular to the bands (top)<ref name="mindat" />]]
The chalcedony in agate is said to be ''fibrous'', forming chain-like "fibers" of intergrown crystals.<ref name="mindat" />{{sfn|Götze|Möckel|Pan|2020|loc=4.1.1}} These fibers are twisted along their length,<ref>{{cite journal |last1=Wang|first1=Yifeng |last2=Merino|first2=Enrique |date=1990-06-01 |title=Self-organizational origin of agates: Banding, fiber twisting, composition, and dynamic crystallization model |journal=[[Geochimica et Cosmochimica Acta]] |language=en |volume=54 |issue=6 |pages=1627–1638 |doi=10.1016/0016-7037(90)90396-3 |bibcode=1990GeCoA..54.1627W |issn=0016-7037}}</ref>{{sfn|Götze|Möckel|Pan|2020|loc=4.1.1}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=11}} giving them a [[helix|helical]] shape.<ref name="PSU" /> Individual fibers were determined by X-ray diffraction to be 0.1-1.0 [[Micrometre|µm]] in diameter and up to several millimeters long.{{sfn|Götze|Möckel|Pan|2020|loc=4.1.1}} There are two different types of chalcedony fibers: length-slow (also known as ''quartzine'') and length-fast.<ref name="chalcedony" />{{sfn|Götze|Möckel|Pan|2020|loc=4.1.1}} Agate primarily contains length-fast chalcedony fibers, consisting of crystals stacked perpendicular to the c-axis (side to side). Agate rarely contains quartzine, consisting of quartz crystals stacked parallel to the c-axis (tip to tip).<ref name="mindat" /><ref name="chalcedony">{{cite web |title=Chalcedony |url=https://www.mindat.org/min-960.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=18 September 2025}}</ref>{{sfn|Götze|Möckel|Pan|2020|loc=4.1.1}} Quartzine only occurs in the outer layer of agates or as thin intergrowths between layers of length-fast chalcedony fibers and macrocrystalline quartz.{{sfn|Götze|Möckel|Pan|2020|loc=4.1.1}}


=== Wall-banded agates ===
===Banding===
''Fortification agates'' have very tight, well-defined bands. They get their name from their appearance resembling the walls of a [[Fortification|fort]]. Fortification agates are one the most common varieties, and they are what most people think of when they hear the word "agate".<ref name="lynch fortification">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=22–25}}</ref>
[[File:Agate Rio Grande do Sul Caillois Donation MNHN n01.jpg|thumb|Agate exhibiting wall banding (top) and level banding (bottom)]]
Agates are broadly separated into two categories based on the type of banding they exhibit:<ref name="mindat" />{{sfn|Lynch|Lynch|2012|pp=34-37}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=19}}


''Lace agates'' exhibit a [[lace]]-like pattern of bands with many swirls, eyes, bends, and zigzags. Unlike most agates, they usually form in [[Vein (geology)|veins]] instead of [[Nodule (geology)|nodules]].<ref name="pabian" />{{rp|p=31}}
#''Wall banding''{{efn|Also called ''concentric banding'',<ref name="mindat" />{{sfn|Lynch|Lynch|2012|pp=22-25}} ''adhesional banding'', or ''fortification banding''{{sfn|Lynch|Lynch|2012|pp=22-25}}}} occurs when agate bands roughly follow the shape of the cavity they formed in. In wall-banded agates, chalcedony fibers grow radially from the cavity walls inward, perpendicular to the direction of the bands.<ref name=
"mindat">{{Cite web |title=Agate |url=https://www.mindat.org/min-51.html |access-date=10 February 2025 |website=mindat.org |publisher=Hudson Institute of Mineralogy}}</ref>{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=19,24}}
#''Level banding''{{efn|Also called ''water-level banding,'' ''gravitational banding'', ''parallel banding'',{{sfn|Lynch|Lynch|2012|pp=34-37}} ''horizontal banding'', or ''Uruguay-type banding''<ref name="mindat" />{{sfn|Lynch|Lynch|2012|pp=34-37}}}} occurs when agate bands form in straight, parallel lines. In level-banded agates, chalcedony precipitates out of solution in the direction of gravity, resulting in horizontal layers of microscopic chalcedony [[Spherulite|spherulites]].<ref name="mindat" />{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=19}}


''Faulted agates'' occur when agate bands are broken and slightly shifted by rock movement and then re-cemented together by chalcedony. They have the appearance of rock layers with [[Fault (geology)|fault lines]] running through them. ''Brecciated agates'' have also had their bands broken apart and re-cemented with chalcedony, but they consist of disjointed band fragments at random angles.<ref name="lynch brecciated">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=70–73}}</ref><ref name="pabian" />{{rp|pp=24,28}} They are a form of [[breccia]], which is a textural term for any rock composed of angular fragments.<ref name="lynch brecciated" /><ref>{{Cite web |title=Brecciated agate |url=https://www.mindat.org/min-7593.html |access-date=February 11, 2025 |website=Mindat.org |publisher=Hudson Institute of Mineralogy}}</ref>
Level banding is less common and usually occurs together with wall banding.<ref name="mindat" /> Wall-banded agate is more fibrous than level-banded agate, which is more granular in texture. Consequently, wall-banded agate is denser and more compact.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=19}}


''Eye agates'' have one or more circular, concentric rings on their surface.<ref>{{Cite web |title=Eye Agate |url=https://www.mindat.org/min-7598.html |access-date=February 11, 2025 |website=Mindat.org |publisher=Hudson Institute of Minerology}}</ref> These "eyes" are actually [[Hemisphere (geometry)|hemispheres]] that form on the husk of the agate and extend inward like a bowl.<ref name="lynch eyes">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=82–85}}</ref> ''Tube agates'' contain tunnel-like structures that extend all the way through the agate.<ref name="lynch tubes">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=64–67}}</ref> These "tubes" may sometimes be banded or hollow, or both. Both tube and eye agates form when chalcedony grows around a needle-shaped crystal of another mineral embedded within the agate, forming [[Stalactite|stalactitic]] structures. Visible "eyes" can also appear on the surface of tube agates if a cut is made (or the agate is weathered) perpendicular to the stalactitic structure.<ref name="pabian" />{{rp|pp=27,35}}
==Formation==
[[File:Agate- & quartz-lined geode 5 (32375570960).jpg|thumb|Agate geode slice with macrocrystalline quartz and a hollow center]]
[[File:Agate Rio Grande do Sul Caillois Donation MNHN n26.jpg|thumb|Agate with level banding in the center surrounded by layers of wall banding]]


''Dendritic agates'' have dark-colored, fern-like patterns ([[Dendrite (crystal)|dendrites]]) on the surface or the spaces between bands.<ref name="lynch dendritic">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=78–81}}</ref><ref name="pabian" />{{rp|p=25}} They are composed of manganese or iron oxides. ''[[Moss agate]]s'' exhibit a [[moss]]-like pattern and are usually green or brown in color. They form when dendritic structures on the surface of an agate are pushed inward with the silica gel during their formation. Moss agate was once believed to be petrified moss, until it was discovered the moss-like formations are actually composed of [[celadonite]], [[hornblende]], or a [[Chlorite group|chlorite]] mineral. ''Plume agates'' are a type of moss agate, but the dendritic "plumes" form tree-like structures within the agate. They are often bright red (from inclusions of [[hematite]]) or bright yellow (from inclusions of [[goethite]]).<ref name="pabian" />{{rp|pp=32–33}} While dendrites frequently occur in banded agates, moss and plume agates usually lack bands altogether. Therefore, they are not true agates according to the mineralogical definition.<ref name="mindat" /><ref>{{Cite web |last=Team |first=Geology In |title=Moss Agate: Formation, Occurrence, Uses |url=https://www.geologyin.com/2020/02/what-is-moss-agate.html#google_vignette |access-date=2025-02-11 |website=Geology In |language=en}}</ref>
Geologists generally understand the early stages of agate formation, but the specific processes that result in band development are widely debated. Since they form in cavities within host rock, agate formation cannot be directly observed,{{sfn|Lynch|Lynch|2012|pp=11-13}} and agate banding has never been successfully replicated in the lab.<ref name="PSU">{{cite web |last1=Brown |first1=Nancy Marie |title=How Do Agates Form? |url=https://www.psu.edu/news/research/story/how-do-agates-form |website=psu.edu |publisher=The Pennsylvania State University |date=31 August 2001|access-date=3 March 2025}}</ref><ref name="moxon 2017">{{cite journal |last1=Moxon |first1=Terry |title=A re-examination of water in agate and its bearing on the agate genesis enigma |journal=Mineralogical Magazine |date=October 2017 |volume=81 |issue=5 |pages=1223–1244 |doi=10.1180/minmag.2017.081.002 |bibcode=2017MinM...81.1223M |url=https://www.cambridge.org/core/journals/mineralogical-magazine/article/abs/reexamination-of-water-in-agate-and-its-bearing-on-the-agate-genesis-enigma/81967BDA3CFFBCE5CD23385B8BF47752 |url-access=subscription |access-date=16 October 2025}}</ref>


''Iris agates'' have bands that are so microscopically fine that when thinly sliced, they cause white light to be [[Diffraction|diffracted]] into its [[spectrum|spectral]] colors. This "iris effect" usually occurs in colorless agates, but it can also occur in brightly colored ones.<ref name="pabian" />{{rp|p=30}}
===Nodular agate===
Agates are most commonly found as [[Nodule (geology)|nodules]] within [[Volcanic rock|volcanic rocks]].<ref name="Moxon 2006">{{Cite journal |last1=Moxon |first1=T |last2=Reed |first2=S. J. B. |year=2006 |title=Agate and chalcedony from igneous and sedimentary hosts aged from 13 to 3480 Ma: a cathodoluminescence study |url=https://pubs.geoscienceworld.org/gsa/minmag/article/70/5/485/140307 |url-status=live |journal=Mineralogical Magazine |volume=70 |issue=5 |pages=485–498 |bibcode=2006MinM...70..485M |doi=10.1180/0026461067050347 |s2cid=54607138 |archive-url=https://web.archive.org/web/20220313041353/https://pubs.geoscienceworld.org/gsa/minmag/article/70/5/485/140307 |archive-date=March 13, 2022 |access-date=October 1, 2006|url-access=subscription }}</ref>{{sfn|Götze|Möckel|Pan|2020|loc=3}} In [[mafic]] rocks such as [[basalt]] and [[andesite]], they form inside cavities called [[Vesicle (geology)|''vesicles'']] (''amygdaloids''{{efn|''Amygdaloid'' means "almond-shaped," but they may also be round, irregular, flat, or bun-shaped.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=12}}}} when filled),{{sfn|Götze|Möckel|Pan|2020|loc=5.1}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=12,50}} gas bubbles that were trapped inside the lava when it cooled.{{sfn|Lynch|Lynch|2012|pp=11-13}}{{sfn|Götze|Möckel|Pan|2020|loc=5.1}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=12}} Since mafic lavas are poor in free silica, there are multiple theories of where the silica originates from, including micro-shards of silica glass from [[volcanic ash]] or [[tuff]] deposits and decomposing plant or animal matter.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=11}} In [[felsic]] rocks such as [[rhyolite]] and rhyolitic [[tuff]], agates form in [[spherulite]]-filled cavities called ''[[lithophysae]]'', commonly called ''[[Thunderegg|thunder eggs]]'' when filled with agate.{{sfn|Götze|Möckel|Pan|2020|loc=3}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=38,49}}


''Sagenitic agates'', or ''sagenites'', have [[Acicular (crystal habit)|acicular]] (needle-shaped) inclusions of another mineral, usually [[anhydrite]], [[aragonite]], [[goethite]], [[rutile]], or a [[zeolite]]. Chalcedony often forms tubes around these crystals and may eventually replace the original mineral, resulting in a [[pseudomorph]].<ref name="pabian" />{{rp|p=34}} The term "sagenite" was originally a name for a type of rutile, and later [[rutilated quartz]]. It has since been used to describe any quartz variety with acicular inclusions of any mineral.<ref>{{cite web |title=Sagenite |url=https://www.mindat.org/min-8578.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=16 February 2025}}</ref>
The cavities are filled with hot, silica-rich water from the surrounding environment, forming a silica gel. This gel crystallizes through a complex process to form agates.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=12,15}} Agates are much harder than the rocks they form in; some varieties (e.g. [[Lake Superior agate]]s) are frequently found detached from their host rock.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=84}}


=== Level-banded agates ===
In mafic rocks, the vesicle walls are often coated with thin layers of [[celadonite]] or [[chlorite]],{{sfn|Lynch|Lynch|2012|pp=11-13}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=18}} soft, green [[phyllosilicate]] minerals that form from the reaction of hot, silica-rich water with the rock.{{sfn|Lynch|Lynch|2012|pp=11-13}} This coating provides a rough surface for the chalcedony fibers to form on, initially as radial [[spherulite]]s. The rough surface also causes agate husks to have a pitted appearance once the coating has been weathered away or removed.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=18-19}} Sometimes, the spherulites grow around mineral inclusions, resulting in eyes, tubes, and sagenitic agates.<ref name="mindat" />
Agates with level banding are traditionally called ''[[onyx]]'', although the formal definition of the term onyx refers to color pattern, not the shape of the bands.<ref>{{cite web |title=Onyx |url=https://www.mindat.org/min-2999.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=February 9, 2025}}</ref> Accordingly, the name "onyx" is also used for wall-banded agates. Onyx is also frequently misused as a name for banded [[calcite]]. The name originates from the Greek word for the human nail, which has parallel ridges.<ref name="pabian">{{Cite book |last=Pabian |first=Roger |title=Agates: Treasures of the Earth |last2=Jackson |first2=Brian |last3=Tandy |first3=Peter |last4=Cromartie |first4=John |date=2016 |publisher=Firefly Books |isbn=978-1-77085-644-8}}</ref>{{rp|p=37}} Typically, onyx bands alternate between black and white or other light and dark colors. ''Sardonyx'' is a variety with red-to-brown bands alternated with either white or black bands.<ref>{{cite web |title=Sardonyx |url=https://www.mindat.org/min-7604.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date= February 10, 2025}}</ref>


[[Thunderegg|Thunder eggs]] are frequently level-banded, however they may also have wall banding. Level banding is also common in [[Lake Superior agate]]s.<ref name="lynch water-level">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=34–37}}</ref>
The first layer of spherulitic chalcedony is typically clear, followed by successive growth bands of chalcedony alternated with chemically precipitated color bands, primarily [[iron oxides]].{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=13}} The center is often macrocrystalline quartz,{{sfn|Lynch|Lynch|2012|pp=11-13}} which can also occur in bands and possibly forms when there is not enough chemically bound water in the silica gel to promote chalcedony [[polymerization]].{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=18}} When the silica concentration of the gel is too low, a hollow center forms, called an agate ''geode''. In geodes, quartz forms crystals around the cavity, with the apex of each crystal pointing towards the center. Occasionally, quartz in agates may be colored, occurring in [[Mineral variety|varieties]] such as [[amethyst]] or [[smoky quartz]].{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=17}}


<gallery mode="packed" heights="130px">
Level bands usually form at the base of the vesicle or in the center when the gel stops adhering to the vesicle walls. This is probably due to a decrease in bound water in the gel.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=19}}
Agate 6 (32375570980).jpg|Agate with both wall banding (top) and level banding (bottom)
 
Less commonly, agates can form as nodules within sedimentary rocks such as [[limestone]], [[Dolomite (rock)|dolomite]] or tuff. These agates form when silica replaces another mineral, or silica-rich water fills cavities left by decomposed plant or animal matter.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=11-12}}
 
===Vein agate===
Agates can also form within rock fissures, called ''veins''.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=11-12}} Vein agates form in a manner similar to nodular agates (see above),{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=13}} and they include lace agates such as [[blue lace agate]] and crazy lace agate. Veins may form in either volcanic rock or [[Sedimentary rock|sedimentary]] rock.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=50}}
 
===Fossil agate===
''Fossil agates'' form when silica replaces the original composition of an organic material.<ref>{{cite web |title=Fossil Agate |url=https://www.mindat.org/min-7603.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=9 March 2025}}</ref> This process is called [[silicification]], a form of [[petrification]]. Examples include [[petrified wood]],<ref>{{cite web |title=Petrified Wood |url=https://www.mindat.org/min-8018.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=9 March 2025}}</ref> agatized coral,<ref>{{cite web |title=Agatized coral |url=https://www.mindat.org/min-43510.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=9 March 2025}}</ref> and Turritella agate (''[[Elimia tenera]]'').<ref name="turritella" /> Although these fossils are often referred to as being "agatized", they are only true agates if they contain bands.<ref name="mindat" />
 
==Variations==
Besides the two fundamental types of banding (''wall'' and ''level''), agates can exhibit a wide variety of band patterns, internal structures, and optical effects.<ref name="mindat" />{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=24-39}}
 
===Band patterns===
*{{anchor|Fortification agate}}''Fortification agates'' are any wall-banded agates with sharp, well-defined bands.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=29}}<ref>{{cite web |title=Fortification Agate |url=https://www.mindat.org/min-7602.html |website=mindat.org |access-date=15 May 2026}}</ref> They are named for their appearance, which resembles the walls of a [[Fortification|fort]]. Some authors do not make a distinction between fortification agates and other wall-banded agates.{{sfn|Lynch|Lynch|2012|pp=22-25}}
 
*{{anchor|Lace agate}}''Lace agates'' are vein agates that exhibit a [[lace]]-like pattern of bands with many swirls, eyes, bends, and zigzags.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=31}}
 
*{{anchor|Faulted agate}}''Faulted agates'' have bands that were broken and slightly shifted by rock movement and then re-cemented together by chalcedony. They have the appearance of rock layers with [[Fault (geology)|fault lines]] running through them.{{sfn|Lynch|Lynch|2012|pp=86-89}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=24,28}}
 
*''Brecciated agates'' also have bands that were broken apart and re-cemented with chalcedony, but they consist of disjointed band fragments at random angles.{{sfn|Lynch|Lynch|2012|pp=70-73}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=24,28}} They are a form of [[breccia]], which is a textural term for any rock composed of angular fragments.{{sfn|Lynch|Lynch|2012|pp=70-73}}<ref>{{Cite web |title=Brecciated agate |url=https://www.mindat.org/min-7593.html |access-date=February 11, 2025 |website=Mindat.org |publisher=Hudson Institute of Mineralogy}}</ref>
 
*{{anchor|Eye agate}}''Eye agates'' have one or more circular, concentric rings on their surface.<ref>{{Cite web |title=Eye Agate |url=https://www.mindat.org/min-7598.html |access-date=February 11, 2025 |website=Mindat.org |publisher=Hudson Institute of Mineralogy}}</ref> These "eyes" are actually [[Hemisphere (geometry)|hemispheres]] that form on the husk of the agate and extend inward like a bowl.{{sfn|Lynch|Lynch|2012|pp=82-85}}
 
*''[[Onyx]]'' is the traditional name for agates with level banding, although the formal definition of the term onyx refers to color pattern, not the shape of the bands.<ref>{{cite web |title=Onyx |url=https://www.mindat.org/min-2999.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=February 9, 2025}}</ref> Accordingly, the name ''onyx'' is also used for wall-banded agates. ''Onyx'' is frequently misused as a name for banded [[calcite]]. The name originates from the Greek word for the human nail, which has parallel ridges.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=37}} Typically, onyx bands alternate between black and white or other light and dark colors.
*''Sardonyx'' is a variety of onyx with red-to-brown bands alternated with either white or black bands.<ref>{{cite web |title=Sardonyx |url=https://www.mindat.org/min-7604.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date= February 10, 2025}}</ref>
 
<gallery mode="packed">
Crazy Lace Agate (Mexico) (32745236336).jpg|Crazy lace agate from Mexico
Agate Braziilia.jpg|Brazilian agate with classic fortification banding
Agate Braziilia.jpg|Brazilian agate with classic fortification banding
Crazy Lace Agate 06.jpg|Crazy lace agate
Eyeballed by all the eye agates (27395607964).jpg|Tumbled [[Lake Superior agate|Lake Superior eye agates]]
Eyeballed by all the eye agates (27395607964).jpg|Tumbled Lake Superior eye agates
Agate, amethyste, quartz.jpg|Faulted and brecciated agate and quartz from [[Maramureș]]
Detail, Dendritic agate (cropped).jpg|Dendritic agate from India
Onyx Mainzer Becken.jpg|Onyx from Germany
Four moss agate cabochons.jpg|Moss agate cabochons
Iris Agate from (Agatized Petrified Wood), Stinking Water, Oregon detail, from- Oregon004 (cropped).jpg|Iris agate from petrified wood
Agate D Bruyere.jpg|Level-banded agate
Onyx Mainzer Becken.jpg|Onyx agate
Thunder Egg Agate (Priday Blue Bed, John Day Formation, Miocene; near Madras, Oregon, USA) 5.jpg|Level-banded thunder egg from Oregon, USA
</gallery>
</gallery>


== Varieties (by locality) ==
===Internal structures===
Agates are very common, and they have been found on every continent, including Antarctica. In addition to names used to describe their structure, numerous geological, local, and trade names are applied to agates from different localities.<ref name="pabian" />{{rp|pp=7–9}} Below is a list of known agate localities and the names of the agates that are found there. This list is not exhaustive.
*{{anchor|Sagenitic agate}}''Sagenitic agates'', or ''sagenites'', have [[Acicular (crystal habit)|acicular]] (needle-shaped) inclusions of another mineral, usually [[anhydrite]], [[aragonite]], goethite, [[rutile]], or a [[zeolite]]. Chalcedony often forms tubes around these crystals and may eventually replace the original mineral, resulting in a [[pseudomorph]].{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=34}} The term "sagenite" was originally a name for a type of rutile, and later [[rutilated quartz]]. It has since been used to describe any quartz variety with acicular inclusions of any mineral.<ref>{{cite web |title=Sagenite |url=https://www.mindat.org/min-8578.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=16 February 2025}}</ref>
 
=== Africa ===
* ''Blue lace agate'' is a pale blue and white lace agate found primarily in [[Namibia]]. These agates formed in dolomite associated with igneous rock.
* ''Botswana agates'' are found in [[basalt]]ic rocks of the [[Permian]] age in [[Botswana]]. They feature contrasting bands of purple, pink, black, grey, and white. Like Lake Superior agates, they are typically small, averaging <!--CheckU-->{{convert|2.5|–|5|cm|in|abbr=on}} in diameter.
* ''Malawi agates'' are typically bright red or orange with contrasting white bands, but some are pink and blue. They can be found in [[Malawi]], and they likely formed in volcanic rock of Permian age.
* Agates have also been found in [[Egypt]], [[Madagascar]], [[South Africa]], and [[Zimbabwe]].<ref name="pabian" />{{rp|pp=131–137}}
<gallery mode="packed" heights="130px">
Blue Lace Agate from Namibia (polished).jpg|Blue lace agate
Agat - Bobonong, Botswana.jpg|Botswana agate
Malawi Agate (Malawi, southeastern Africa) (32734668126).jpg|Malawi agate
</gallery>


=== Antarctica ===
*{{anchor|Tube agate}}''Tube agates'' contain tunnel-like structures that extend all the way through the agate.{{sfn|Lynch|Lynch|2012|pp=64-67}} These "tubes" may sometimes be banded or hollow, or both. Tube agates form when chalcedony grew around sagenitic inclusions embedded within the agate, forming [[Stalactite|stalactitic]] structures. Visible "eyes" can also appear on the surface of tube agates if a cut is made (or the agate is weathered) perpendicular to the stalactitic structure.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=27,35}}
* White and clear banded agates have been found by scientists at [[Bellingshausen Station]], a Russian outpost on [[King George Island (South Shetland Islands)|King George Island]].<ref>{{cite web |title=Collins Harbour, King George Island, South Shetland Islands, Antarctic Peninsula, Western Antarctica, Antarctica |url=https://www.mindat.org/loc-420174.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=16 February 2025}}</ref>
[[File:Agate (Tertiary; Agate Bay, King George Island, South Shetland Islands) 1.jpg|thumb|center|215px|Agate from King George Island, Antarctica]]


=== Asia ===
*{{anchor|Dendritic agate}}''Dendritic agates'' have dark-colored, fern-patterned inclusions ([[Dendrite (crystal)|dendrites]]) that form on the surface of agates or in the spaces between bands.{{sfn|Lynch|Lynch|2012|pp=78-81}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=25}}<ref>{{cite web |title=Dendritic Agate |url=https://www.mindat.org/min-7599.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=15 May 2026}}</ref> They are composed of [[manganese oxide]]s or [[iron oxide]]s.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=25}}
* India has produced agates since as early as the 11th century. These include carnelian agates, moss agates, and dendritic agates.
*''[[Moss agate]]s'' exhibit a [[moss]]-like inclusions and are usually green or brown in color.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=32-33}}<ref name="moss">{{cite web |title=Moss Agate |url=https://www.mindat.org/min-2791.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=15 May 2026}}</ref> They form when dendritic structures on the surface of an agate are pushed inward with the silica gel during their formation. Moss agate was once believed to be petrified moss,{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=32-33}} until it was discovered the moss-like formations are actually composed of celadonite, [[hornblende]], or a [[Chlorite group|chlorite]] mineral.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=32-33}}<ref name="moss" />  
* Yemen is home to a variety of agate called ''mocha stone'', named after the port city of [[Mokha|Mocha]] (also spelled Mokha or Mukha) on the [[Red Sea]]. These agates likely formed in [[tuff]] deposits of [[Chattian|Late Oligocene]] and [[Early Miocene]] age.
*''Plume agates'' are a type of moss agate, but the dendritic "plumes" form tree-like structures within the agate. They are often bright red (from inclusions of [[hematite]]) or bright yellow (from inclusions of [[goethite]]).{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=32-33}}<ref>{{cite web |title=Plume Agate |url=https://www.mindat.org/min-26405.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=15 May 2026}}</ref> Both moss and plume agates usually lack any form of banding, and therefore are not true agates according to the mineralogical definition.<ref name="mindat" /><ref>{{Cite web |last=Team |first=Geology In |title=Moss Agate: Formation, Occurrence, Uses |url=https://www.geologyin.com/2020/02/what-is-moss-agate.html#google_vignette |access-date=2025-02-11 |website=Geology In |language=en}}</ref>
* Agates have also been found in [[Iran]], [[Mongolia]], [[China]],<ref name="pabian" />{{rp|pp=138–143}} and [[Russia]].<ref name="mindat" />
[[File:Agates Mandalgobi (2).jpg|thumb|center|180px|Rough agates from the [[Gobi Desert]] in Mongolia]]


=== Australia ===
*[[Enhydro agate|''Enhydro agates'']], or ''enhydros'', form when bubbles of liquid water become trapped within an agate (or chalcedony) nodule or geode, often long after its formation.<ref>{{cite book |last1=Bates |first1=R. L. |last2=Jackson |first2=J. A. |title=Glossary of Geological Terms |date=1987 |publisher=American Geological Institute |location=Alexandria, Virginia |page=788 |edition=3rd |url=https://www.gamineral.org/writings/enhydros-gray.html |access-date=9 March 2025}}</ref><ref>{{cite web |title=Enhydro Agate |url=https://www.mindat.org/min-7596.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=9 March 2025}}</ref>
* ''Queensland agates,'' found in the State of [[Queensland]], often occur in colors that are rarely found in agates from other regions, such as green and yellow-green. They formed in basaltic lava flows of the Late Permian period. Level banding is common in Queensland agates, while inclusions are uncommon. Queensland is also home to several kinds of ''thunder egg'', which are thought to date from the [[Early Cretaceous]] period.
* Agates have also been found in [[Tasmania]] and other regions of Australia.<ref name="pabian" />{{rp|pp=144–149}}
[[File:Agat ze strefą krustalną - Agate Creek, Queensland, Australia.jpg|thumb|center|215px|Queensland agate with level banding]]


=== Europe ===
<gallery mode="packed">
* Agate was discovered in [[Sicily]] by the Greek scholar Theophrastus in 350 BC. At the time, Sicily was a colony of [[ancient Greece]]. The name "agate" comes from the Achates River, the Greek name for what is now known as the [[Dirillo|Dirillo River]]. Agates in Sicily formed in balsaltic lavas and pyroclastic rocks of the [[Pliocene|Pilocene]] epoch.
Agate Rio Grande do Sul Caillois Donation MNHN n24.jpg|Sagenitic agate from Brazil with numerous needle-like inclusions
* [[Germany]] is a well-known historic source of agate. Agates mined from volcanic rock of the Permian period have been processed in [[Idar-Oberstein]] since at least 1375, but possibly as early as the [[Roman Empire]]. Agates from the Idar-Oberstein area are often red and pink, but other colors have also been observed. Many museum specimens include features such as eyes, tubes, moss, plumes, and sagenite.
Detail, Dendritic agate (cropped).jpg|Close-up of a dendrite in an agate from India
* [[Scotland]] is an abundant source of a wide variety of agates. There are at least 50 main agate localities in Scotland. Scottish agates have been popular in jewelry for several hundred years, particularly during the [[Victorian era]]. They formed in two types of rock: andesite from the [[Early Devonian]] period and basalt from the [[Tertiary (period)|Tertiary]] period. The andesite deposits are more significant and extend from [[Stonehaven]] in the northeast to just south of [[Ayr]] in the southwest. The basaltic agates are confined to the islands off the west coast of Scotland and are collectively called the ''Small Isles agates''. The colors of Scottish agates vary, and bands may be different shades of blue, grey, purple, pink, brown, orange, or red.
Four moss agate cabochons.jpg|Moss agate [[cabochons]]
* ''Pot stones'' or ''potato stones'' are irregular agate nodules or geodes found in [[Bristol]] and [[Somerset]], England. They typically consist of a reddish, banded agate surrounding a hollow cavity lined with macroscopic quartz, although some are completely filled with agate. Other varieties of agate have also been found elsewhere in England.
* Agates can also be found in [[Wales]], the [[Czech Republic]], [[Poland]], [[France]],<ref name="pabian" />{{rp|pp=52–75}} and many other European countries.<ref name="mindat" />
<gallery mode="packed" heights="130px">
Agate-MCG 91225-P4150832-white.jpg|German agate from Idar-Oberstein
Agate detail, Scotland 007 (cropped).jpg|Close-up of a Scottish agate from [[Ayrshire]]
Quartz-agate (12250382174).jpg|Potato stone from England
Agate from Czech Republic (7).jpg|Agate from Czech Republic
</gallery>
</gallery>


=== North America ===
===Optical effects===
* ''Boley agates'' are sedimentary agates of [[Pennsylvanian (geology)|Pennsylvanian]] age. They are found in the Boley conglomerate layer of the Vamoosa formation in central [[Oklahoma]]. Boley agates commonly occur with brecciated chert.<ref>{{cite journal |last1=Suneson |first1=Neil H. |last2=Lyon |first2=William G. |last3=Goza |first3=David |title=Boley Agate — Chert Breccia Clasts In The Vamoosa  Formation |journal=Shale Shaker |date=July–August 2013 |volume=64 |issue=1 |pages=22–37 |url=http://www.ogs.ou.edu/geology/pdf/BOLEYAGATEnhs.pdf |access-date=27 July 2025}}</ref>
*{{anchor|Iris agate}}''Iris agates'' have bands fine enough that when thinly sliced, they cause transmitted light to be [[Diffraction|diffracted]] into its [[spectrum|spectral]] colors.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=30}}<ref>{{cite web |title=Iris Agate |url=https://www.mindat.org/min-7610.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=15 May 2026}}</ref> This "iris effect" usually occurs in colorless agates, but it can also occur in brightly colored ones.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=30}}
* ''Coldwater agates'', such as the [[Lake Michigan]] cloud agate, are sedimentary agates that formed within [[limestone]] and [[Dolomite (rock)|dolomite]] strata of marine origin. Like volcanic agates, Coldwater agates formed from silica gels that lined pockets and seams within the bedrock. These agates are typically less colorful, with banded lines of grey and white chalcedony.<ref>{{Cite book|last=Garvin|first=Paul|url=https://books.google.com/books?id=3a1XkpBGdAcC&q=Coldwater+agates&pg=PA65|title=Iowa's Minerals: Their Occurrence, Origins, Industries, and Lore|date=2010-09-13|publisher=University of Iowa Press|isbn=978-1-60938-014-4|language=en|access-date=2020-10-29|archive-date=2023-08-26|archive-url=https://web.archive.org/web/20230826164756/https://books.google.com/books?id=3a1XkpBGdAcC&q=Coldwater+agates&pg=PA65|url-status=live}}</ref>
* ''Crazy lace agate'' is a brightly colored lace agate from [[Mexico]] with a complex pattern, demonstrating randomized distribution of contour lines and circular droplets, scattered throughout the rock. The stone is typically colored red and white but is also seen to exhibit yellow and grey combinations as well.<ref>{{Cite book|last1=Atkinson|first1=Bill|url=https://books.google.com/books?id=ZSP94tREw7AC&q=Crazy+lace+agate&pg=PA165|title=Within the Stone: Photography|last2=Ackerman|first2=Diane|date=2004|publisher=BrownTrout Publishers|isbn=978-0-7631-8189-5|language=en|access-date=2020-10-29|archive-date=2023-08-26|archive-url=https://web.archive.org/web/20230826164751/https://books.google.com/books?id=ZSP94tREw7AC&q=Crazy+lace+agate&pg=PA165|url-status=live}}</ref> Crazy lace agate is a vein agate that formed in sedimentary rock of the late [[Cretaceous]] period.<ref name="pabian" />{{rp|p=121}}
* ''Dugway geodes'' are a type of thunder egg found in [[Utah]]. They are typically light grey to blue and often contain hollow cavities lined with drusy quartz.<ref name="pabian" />{{rp|p=92}}
* [[Fairburn Agate|''Fairburn agates'']] are rare fortification agates named for [[Fairburn, South Dakota]]. They are sedimentary agates that originated during the [[Pennsylvanian (geology)|Pennsylvanian]] period, and then weathered from their host rock and redeposited during the [[Oligocene]] epoch in parts of South Dakota and [[Nebraska]].<ref>{{cite web |title=Fairburn Agate |url=https://www.mindat.org/min-1441.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=16 February 2025}}</ref>
* ''Laguna agate'' is a brightly colored agate variety that was first discovered in Ojo Laguna, Chihuahua, Mexico.<ref>{{Cite web |title=Laguna Agate |url=https://www.mindat.org/min-7611.html |access-date=2025-02-16 |website=www.mindat.org}}</ref> It features vibrant bands in shades of red, orange, pink, or purple. Laguna agates formed in [[andesite]] and are geologically young. They frequently contain inclusions and many exhibit parallax or shadow banding.<ref name="pabian" />{{rp|pp=114–115}}
* ''[[Lake Superior agate]]s'' are believed to be the world's oldest agates;<ref>{{cite web |title=Lake Superior Agate |url=https://www.mindat.org/min-9253.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=16 February 2025}}</ref> they formed as nodules in basalt up to 1.2 billion years ago during the Late [[Precambrian]]. These agates are primarily found near the shores of [[Lake Superior]] in the [[United States|U.S.]] states of [[Minnesota]], [[Michigan]], and [[Wisconsin]], and in the [[Canada|Canadian]] province of [[Ontario]]. They are not named after the lake, but rather the Lake Superior [[Till]], the [[Pleistocene]] glacial deposit in which they are found.<ref name="pabian" />{{rp|pp=83–84}} This deposit also extends into portions of [[Iowa]], [[Nebraska]], [[Kansas]], and [[Missouri]], and Lake Superior agates have been carried south by the [[Mississippi River]] into [[Arkansas]] and [[Louisiana]]. Lake Superior agates have bands in shades of red, orange, yellow, brown, white, and grey. They can contain a variety of structural features, including eyes, tubes, sagenite, dendrites, faults, and geodes.<ref name="lynch whole book">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6}}</ref>
* ''Lysite agate'' is a vein agate named after Lysite Mountain, [[Wyoming]]. It is frequently colorful and may contain moss and plumes in addition to bands.<ref name="pabian" />{{rp|p=79}}
* ''Nebraska blue agate'' is a sedimentary agate with dendritic patterns that formed during the Oligocene epoch. It can be found throughout northwestern Nebraska and southwestern South Dakota.<ref name="pabian" />{{rp|p=80}}
* Oregon is known for several different varieties of agate. It is probably best known for its ''thunder eggs'', which form in rhyolitic ash and have a brown rhyolite shell that is usually filled with blue and white agate.<ref name="pabian" />{{rp|p=99}} ''Holley blue agate'' (also spelled "Holly blue agate") is a rare lavender to blue agate found only near [[Holley, Oregon]].<ref name="pabian" />{{rp|p=103}}
* ''[[Patuxent River stone]]'' is a red and yellow form of agate only found in [[Maryland]], where it is the state gem.<ref>{{cite web |title=Maryland State Gem - Patuxent River Stone |url=https://msa.maryland.gov/msa/mdmanual/01glance/symbols/html/gem.html |website=maryland.gov |publisher=Maryland State Archives |access-date=16 February 2025}}</ref>
* ''Sweetwater agates'' are small moss agates found in [[Miocene]] age [[sandstone]] near Sweetwater River, Wyoming. They also contain brown or black dendrites and fluoresce under [[Ultraviolet|UV]] light.<ref name="pabian" />{{rp|p=79}}
* ''Turritella agate'' is a brown fossil agate formed from the remains of an extinct species of freshwater snail ([[Elimia tenera]]) with an elongated spiral shell. The name is a misnomer; it was originally thought to be the fossil of a different genus of gastropods, [[Turritella]]. It is found in the [[Green River Formation]] of Wyoming.<ref name="turritella">{{Cite web |last=King |first=Hobart M. |title=Turritella Agate |url=https://geology.com/gemstones/turritella/ |access-date=16 February 2025 |website=geology.com}}</ref>
* Other varieties of agate have also been found in nearly every U.S. state, northern Mexico, and in the Canadian provinces of [[Nova Scotia]], [[Manitoba]], and [[British Columbia]].<ref name="pabian" />{{rp|pp=76–121}}
<gallery mode="packed" heights="130px">
Crazy Lace Agate - Macro Panorama.jpg|Crazy lace agate
Dugway Geode (Juab County, Utah, USA) 2 (34581522545).jpg|Dugway geode from Utah
Fairburn Agate (ultimately derived from the Minnelusa Formation, Pennsylvanian-Permian; collected east of the Black Hills, western South Dakota, USA) 26 (32406082220).jpg|Fairburn agate from western South Dakota
LagunaAgateFromMexico.jpg|Laguna agate
Agate nodule ("Lake Superior Agate") (floor of Lake Superior, offshore Keweenaw Peninsula, Michigan USA) 2 (33741645898).jpg|Rough Lake Superior agate from [[Keweenaw Peninsula]], Michigan
Thunder Egg Agate (Priday Blue Bed, John Day Formation, Miocene; near Madras, Oregon, USA) 3 (33992544563).jpg|Thunder egg from Oregon
Holley Blue Agate (Linn County, Oregon, USA) 9.jpg|Holley blue agate from Oregon
Elimia fossils Wyoming.jpg|alt=An irregular dark stone with a flat polished front; many white fragments of elongated, spiral, "corkscrew" shells seem to float in the dark stone|Turritella agate (''Elimia tenera'')
Chalcedony (Variety Agate)-262773.jpg|Agate from British Columbia
</gallery>


=== South America ===
*''[[Parallax]] banding'', also called ''shadow banding'', can occur when an agate has very fine, alternating bands of transparent and opaque chalcedony. A shadow is cast onto the opaque bands which appears to move when the agate is viewed from different angles.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=115}}
* ''Brazilian agate'' is probably one of the largest agates. They can reach {{convert|0.9|m|ft|abbr=on}} in diameter and weigh over {{convert|120|kg|lb|abbr=on}}. Brazilian agate is found primarily as nodules and geodes in decomposed volcanic ash and basalt of [[Lopingian|Late Permian]] age. The largest deposits are in the Brazilian state of [[Rio Grande do Sul]], but significant amounts can also be found throughout southeastern [[Brazil]]. Some specimens can be very colorful and include features such as eyes, tubes, dendrites, and sagentite. However, most Brazilian agate that is mined is naturally pale yellow, gray, or colorless and artificially dyed before being brought to market.
* [[Condor agate|''Condor agates'']] are found in the [[Mendoza Province|Mendoza]] province of [[Argentina]]. They typically have bright red and yellow fortification banding and may contain mossy or sagenitic inclusions. Other varieties of agate can also be found in the [[Patagonia]] area of Argentina, including ''crater agate'' (typically hollow nodules with black and red bands) and ''puma agate'' (agatized coral).  
* [[Uruguay]] was the first major source of agates in South America. Agates were discovered there in 1830, but sources in neighboring Brazil became more popular in the late 19th and 20th centuries.
* Agates have also been found in [[Chile]] and [[Peru]].<ref name="pabian" />{{rp|pp=122–131}}


<gallery mode="packed" heights="130px">
<gallery mode="packed">
Brazilian agate section (detail).JPG|Natural Brazilian agate
Iris Agate from (Agatized Petrified Wood), Stinking Water, Oregon detail, from- Oregon004 (cropped).jpg|Iris effect in a thin slice of agatized petrified wood from [[Oregon]]
5agat, Brazylia.jpg|Dyed Brazilian agate
Argentina001.jpg|Condor agate
Achat 1.jpg|Agate from Uruguay
</gallery>
</gallery>
== Local varieties ==
Agates are very common, and they have been found on every continent,{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=5}} including Antarctica.<ref name="Antarctica" /> In addition to the structural varieties detailed in the previous section, numerous geological, local, and trade names are used to describe agates from different localities.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=7-9}} Below is a table of agate varieties from different regions of the world.{{sticky header}}
{| class="wikitable sortable sticky-header" |
|+ Local varieties of agate
|-
! Name !! Locality !! Description !! Type !! Geologic environment !! Age !! Photo(s) !! {{Ref.}}
|-
| (Unnamed agate) || [[Bellingshausen Station]], King George Island, Antarctica || White and clear bands || [[#Nodular agate|Nodular agate]] || || || [[File:Agate (Tertiary; Agate Bay, King George Island, South Shetland Islands) 1.jpg|130px]]<br />Agate from King George Island, Antarctica || <ref name="Antarctica">{{cite web |title=Collins Harbour, King George Island, South Shetland Islands, Antarctic Peninsula, Western Antarctica, Antarctica |url=https://www.mindat.org/loc-420174.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=16 February 2025}}</ref>
|-
| {{anchor|Blue lace agate}}Blue lace agate || Primarily [[Namibia]]; also [[Malawi]], [[Kenya]], [[Turkey]], [[Georgia (country)|Georgia]], and [[Zambia]] || Pale blue and white lace agate || [[#Vein agate|Vein agate]] || Volcanic rock (dolomite associated with [[dolerite]]) || [[Jurassic]] period || [[File:Blue Lace Agate.jpg|130px]]<br />Blue lace agate from Ysterputz Mine, Namibia || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=134-135}}<ref name="JAES">{{cite journal |last1=Welman-Purchase |first1=Megan |last2=Wicht |first2=Joanna |last3=Miller |first3=Duncan |last4=Roelofse |first4=Frederick |title=Blue lace agate and chalcedony pseudomorphs from Ysterputs in southern Namibia |journal=Journal of African Earth Sciences |date=April 2024 |volume=212 |article-number=105211 |doi=10.1016/j.jafrearsci.2024.105211 |bibcode=2024JAfES.21205211W |doi-access=free }}</ref>
|-
| Blue Bed (Pony Butte) thunder egg || Richardson Ranch (formerly Priday Ranch), northeast of [[Madras, Oregon]], United States || Blue and white banding with dark brown shell, frequently level-banded || [[#Nodular agate|Nodular agate]] ([[thunder egg]]) || Volcanic rock (John Day Formation, [[Rhyolite|rhyolitic]] volcanic ash) || [[Miocene]] epoch || [[File:Thunder Egg Agate (Priday Blue Bed, John Day Formation, Miocene; near Madras, Oregon, USA) 3 (33992544563).jpg|130px]]<br />Blue Bed thunder egg from near Madras, Oregon || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=99}}
|-
| {{anchor|Boley agate}}Boley agate || Central [[Oklahoma]], United States || White fortification and eye banding with clasts of brecciated chert || [[#Vein agate|Vein agate]] || Sedimentary rock (Boley conglomerate layer, Vamoosa formation) || [[Virgilian series]] || {{Image requested inline}} || <ref>{{cite journal |last1=Suneson |first1=Neil H. |last2=Lyon |first2=William G. |last3=Goza |first3=David |title=Boley Agate — Chert Breccia Clasts In The Vamoosa Formation |journal=Shale Shaker |date=July–August 2013 |volume=64 |issue=1 |pages=22–37 |url=http://www.ogs.ou.edu/geology/pdf/BOLEYAGATEnhs.pdf |access-date=27 July 2025}}</ref>
|-
| {{anchor|Botswana agate}}Botswana agate || [[Botswana]] || Typically {{convert|2.5|–|5|cm|in|abbr=on}} in diameter, with contrasting bands of purple, pink, black, grey, and white || [[#Nodular agate|Nodular agate]] || Volcanic rock (Karoo Series, basalt) || [[Permian]] period || [[File:Agat - Bobonong, Botswana.jpg|130px]]<br />Botswana agate || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=131-132}}
|-
| [[Brazilian agate]] || [[Rio Grande do Sul]] and other southeastern states, Brazil || Often large, up to {{convert|0.9|m|ft|abbr=on}} in diameter and over {{convert|120|kg|lb|abbr=on}}, commonly pale yellow, gray, or colorless (usually sold artificially dyed), are more colorful or contain structural features || [[#Nodular agate|Nodular agate]] || Volcanic rock (decomposed volcanic ash and basalt) || Late Permian period || [[File:Brazilian agate section (detail) (cropped).JPG|130px]]<br />Natural Brazilian agate <br><br> [[File:5agat, Brazylia.jpg|130px]]<br />Dyed Brazilian agate || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=122-126}}
|-
| [[Condor agate]] || [[Mendoza Province|Mendoza]], Argentina || Bright red and yellow fortification banding, may contain mossy or sagenitic inclusions || [[#Nodular agate|Nodular agate]] || || || [[File:Argentina001 (cropped).jpg|130px]]<br />Condor agate || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=129-131}}
|-
| {{anchor|Coldwater agate}}Coldwater agate (Lake Michigan cloud agate) || [[Great Lakes region|Great Lakes Region]], United States|| Banded lines of grey and white chalcedony || [[#Nodular agate|Nodular agate]] || Sedimentary rock (marine limestone and dolomite) || || {{Image requested inline}} || <ref>{{Cite book|last=Garvin|first=Paul|url=https://books.google.com/books?id=3a1XkpBGdAcC&q=Coldwater+agates&pg=PA65|title=Iowa's Minerals: Their Occurrence, Origins, Industries, and Lore|date=2010-09-13|publisher=University of Iowa Press|isbn=978-1-60938-014-4|language=en|access-date=2020-10-29|archive-date=2023-08-26|archive-url=https://web.archive.org/web/20230826164756/https://books.google.com/books?id=3a1XkpBGdAcC&q=Coldwater+agates&pg=PA65|url-status=live}}</ref>
|-
| {{anchor|Crater agate}}Crater agate || [[Patagonia]], Argentina || Typically hollow, black with red bands near the center || [[#Nodular agate|Nodular agate]] || Volcanic rock (rhyolite) || [[Jurassic]] period || {{Image requested inline}} || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=131}}
|-
| {{anchor|Crazy lace agate}}Crazy lace agate || Mexico || Brightly colored lace agate, typically white and red, sometimes yellow and grey || [[#Vein agate|Vein agate]] || Sedimentary rock || Late [[Cretaceous]] period || [[File:Crazy Lace Agate - Macro Panorama.jpg|130px]]<br />Crazy lace agate || <ref>{{Cite book|last1=Atkinson|first1=Bill|url=https://books.google.com/books?id=ZSP94tREw7AC&q=Crazy+lace+agate&pg=PA165|title=Within the Stone: Photography|last2=Ackerman|first2=Diane|date=2004|publisher=BrownTrout Publishers|isbn=978-0-7631-8189-5|language=en|access-date=2020-10-29|archive-date=2023-08-26|archive-url=https://web.archive.org/web/20230826164751/https://books.google.com/books?id=ZSP94tREw7AC&q=Crazy+lace+agate&pg=PA165|url-status=live}}</ref>{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=121}}
|-
| {{anchor|Crowley's Ridge agate}}Crowley's Ridge agate || [[Crowley's Ridge]], Arkansas and Missouri, United States || Pale yellow to tan fortification banding, rarely pink to red (purportedly due to heating in wildfires). Also found along the Mississippi River basin in Mississippi and Louisiana. || [[#Nodular agate|Nodular agate]] || Sedimentary rock || [[Paleozoic]] era (deposited in [[Pliocene]] gravel) || || <ref>{{cite web |title=Crowley's Ridge Agates |url=https://www.sailorenergy.net/Agates/AgatesArkansasCrowleysRidge01.html |website=Dwarves' Earth Treasures |access-date=17 February 2026}}</ref><ref>{{cite web |title=Agate from Crowley's Ridge (Crowleys Ridge), Malden, Dunklin County, Missouri, USA |url=https://www.mindat.org/locentry-1499833.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=17 February 2026}}</ref><ref>{{cite web |last1=Guccione |first1=M. J. |last2=Prior |first2=W. L. |last3=Rutledge |first3=E. M. |title=The Tertiary and Quaternary Geology of Crowley's Ridge: A Guidebook |url=https://www.geology.arkansas.gov/publication/guide_books/GB-86-4-guidebook.html |website=Office of the State Geologist |publisher=Arkansas Geological Commission |access-date=17 February 2026 |date=1986}}</ref>
|-
| {{anchor|Dugway geode}}Dugway geode || [[Utah]], United States || Light grey to blue, often contain hollow cavities lined with drusy quartz || [[#Nodular agate|Nodular agate]] ([[Thunderegg|thunder egg]]) || || || [[File:Dugway Geode (Juab County, Utah, USA) 2 (34581522545).jpg|130px]]<br />Dugway geode from Utah || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=92}}
|-
| [[Fairburn agate]] || [[South Dakota]] and [[Nebraska]], United States || Red fortification banding || [[#Nodular agate|Nodular agate]] || Sedimentary rock (marine carbonate sediments) || [[Pennsylvanian (geology)|Pennsylvanian]] period || [[File:Fairburn Agate (ultimately derived from the Minnelusa Formation, Pennsylvanian-Permian; collected east of the Black Hills, western South Dakota, USA) 26 (32406082220).jpg|130px]]<br />Fairburn agate from western South Dakota || <ref>{{cite web |title=Fairburn Agate |url=https://www.mindat.org/min-1441.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=16 February 2025}}</ref>{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=77}}
|-
| German agate || Near [[Idar-Oberstein]], Germany || Often red or pink, sometimes other colors || [[#Nodular agate|Nodular agate]] || Volcanic rock || Permian period || [[File:Agate-MCG 91225-P4150832-white.jpg|130px]]<br />German agate from Idar-Oberstein || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=52-55}}
|-
| {{anchor|Holley blue agate|Holly blue agate}}Holley (Holly) blue agate || Near [[Holley, Oregon]], United States || Lavender to blue || [[#Nodular agate|Nodular agate]] || || || [[File:Holley Blue Agate (Linn County, Oregon, USA) 9.jpg|130px]]<br />Holley blue agate from Oregon || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=103}}
|-
| {{anchor|Laguna agate}}Laguna agate || Ojo Laguna, [[Chihuahua (state)|Chihuahua]], Mexico || Vibrant bands in shades of red, orange, pink, or purple, often exhibit parallax or shadow banding, inclusions common || [[#Nodular agate|Nodular agate]] || Volcanic rock (andesite) || [[Tertiary period|Tertiary]] period || [[File:LagunaAgateFromMexico (cropped).jpg|130px]]<br />Laguna agate || <ref>{{Cite web |title=Laguna Agate |url=https://www.mindat.org/min-7611.html |access-date=2025-02-16 |website=www.mindat.org}}</ref>{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=114-115}}
|-
| [[Lake Superior agate]] || Near [[Lake Superior]], United States and Canada || Bands in shades of red, orange, yellow, brown, white, and grey, level banding and various structural features common || [[#Nodular agate|Nodular agate]] || Volcanic rock (basalt) || Late [[Precambrian]] (can be found in [[Pleistocene]] glacial deposits) || [[File:Agate nodule ("Lake Superior Agate") (floor of Lake Superior, offshore Keweenaw Peninsula, Michigan USA) 2 (33741645898).jpg|130px]]<br />Rough Lake Superior agate from [[Keweenaw Peninsula]], Michigan || <ref>{{cite web |title=Lake Superior Agate |url=https://www.mindat.org/min-9253.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=16 February 2025}}</ref>{{sfn|Lynch|Lynch|2012}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=83-84}}
|-
| {{anchor|Lysite agate}}Lysite agate || Lysite Mountain, [[Fremont County, Wyoming]], United States || Colorful bands with plumes and moss || [[#Vein agate|Vein agate]] || Sedimentary rock (marine origin) || || [[File:Lysite Agate Wyoming.jpg|130px]]<br />Lysite agate from Wyoming || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=79}}
|-
| {{anchor|Malawi agate}}Malawi agate || [[Malawi]] || Typically bright red or orange with contrasting white bands, some are pink and blue || [[#Nodular agate|Nodular agate]] || Volcanic rock || Permian period || [[File:Malawi Agate (Malawi, southeastern Africa) (32734668126).jpg|130px]]<br />Malawi agate || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=134}}
|-
| Potato stone (Pot stone) || [[Bristol]] and [[Somerset]], England, United Kingdom || Irregularly-shaped, reddish, banded agate nodules, typically surrounding a hollow cavity lined with macroscopic quartz, but sometimes completely filled || [[#Nodular agate|Nodular agate]] || Sedimentary rock (dolomitic [[Conglomerate (geology)|conglomerate]] and [[marl]]) || [[Triassic]] period || [[File:Quartz-agate (12250382174).jpg|130px]]<br />Potato stone from England || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=72}}
|-
| {{anchor|Puma agate}}Puma agate || [[Andes]], Patagonia, Argentina || Agatized coral || [[#Fossil agate|Fossil agate]] || Sedimentary rock (marine) || || {{Image requested inline}} || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=131}}
|-
| {{anchor|Queensland agate}}Queensland agate || [[Queensland]], Australia || Often green or yellow-green (colors that are rarely found in other regions), frequently level-banded || [[#Nodular agate|Nodular agate]] || Volcanic rock (basaltic lava flows) || Late Permian period || [[File:Agat ze strefą krustalną - Agate Creek, Queensland, Australia.jpg|130px]]<br />Queensland agate with level banding || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=144-145}}
|-
| Scottish agate || [[Stonehaven]] to just south of [[Ayr]], near [[Oban]], and surrounding the [[Cheviot Hills]], Scotland, United Kingdom || Various colored bands || [[#Nodular agate|Nodular agate]] || Volcanic rock (andesite) || [[Early Devonian]] period || [[File:Agate detail, Scotland 007 (cropped).jpg|130px]]<br />Close-up of a Scottish agate from [[Ayrshire]] || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=58-61}}
|-
| Small Isles agate || Islands off the west coast of [[Scotland]], United Kingdom || Various colored bands || [[#Nodular agate|Nodular agate]] || Volcanic rock (basalt) || [[Tertiary (period)|Tertiary]] period || {{Image requested inline}} || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=58-61}}
|-
| {{anchor|Sweetwater agate}}Sweetwater agate || Near [[Sweetwater River (Wyoming)|Sweetwater River]], [[Wyoming]], United States || Small moss agates with brown or black dendrites, fluorescent under [[Ultraviolet|UV]] light || [[#Nodular agate|Nodular agate]] || Sedimentary rock (sandstone) || Miocene epoch || {{Image requested inline}} || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=79}}
|-
| [[Elimia tenera|Turritella agate]] || [[Wyoming]], United States || Brown fossil agate with the elongated spiral shells of an extinct freshwater snail (''Elimia tenera'') || [[#Fossil agate|Fossil agate]] || Sedimentary rock ([[Green River Formation]]) || [[Eocene]] epoch || [[File:Elimia fossils Wyoming.jpg|130px]]<br />Turritella agate (''Elimia tenera'') || <ref name="turritella">{{Cite web |last=King |first=Hobart M. |title=Turritella Agate |url=https://geology.com/gemstones/turritella/ |access-date=16 February 2025 |website=geology.com}}</ref>
|}


== Uses ==
== Uses ==
Agate has long been popular as a [[gemstone]] in [[jewelry]] such as [[pins]], [[brooch]]es, [[necklaces]], [[earrings]], and [[bracelets]]. Agates have also historically been used in the art of [[hardstone carving]] to make knives, [[inkstand]]s, [[seal (emblem)|seals]], [[marbles]], and other objects. Today, they are still used to make beads, decorative displays, carvings, and [[cabochons]], as well as face-polished and tumble-polished specimens of varying size and origin. Agate collecting is a popular hobby, and agate specimens can be found in numerous gift shops, museums, galleries, and private collections.<ref name="pabian" />{{rp|pp=159–168}}
Agate is frequently used as a gemstone in [[jewelry]] such as [[pins]], [[brooch]]es, [[necklaces]], [[earrings]], and [[bracelets]]. Agates have also historically been used in the art of [[hardstone carving]] to make knives, [[inkstand]]s, [[seal (emblem)|seals]], [[marbles]], and other objects. Today, they are widely used to make beads, decorative displays, carvings, and [[cabochons]], as well as face-polished and tumble-polished specimens of varying size and origin. Agate collecting is a popular hobby, and agate specimens can be found in numerous gift shops, museums, galleries, and private collections.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=159-168}}


Industrial uses of agate exploit its hardness, ability to retain a highly polished surface finish and resistance to chemical attack. Historically, it was used to make bearings for highly accurate [[laboratory balance]]s and [[Mortar and pestle|mortars and pestles]] to crush and mix chemicals. During the [[Second World War]], black agate beads mined from [[Queensland, Australia]] were used in the turn and bank indicators of military aircraft.<ref name="pabian" />{{rp|pp=168–169}}
Industrial uses of agate exploit its hardness, ability to retain a highly polished surface finish and resistance to chemical attack. Historically, it was used to make bearings for highly accurate [[laboratory balance]]s and [[Mortar and pestle|mortars and pestles]] to crush and mix chemicals. During the [[Second World War]], black agate beads mined from [[Queensland]], Australia were used in the turn and bank indicators of military aircraft.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=168-169}}


Agates, particularly moss agates, were first used during the [[Stone Age]] to make tools such as arrow and spear points, needles, and hide scrapers. Artefacts from as early as 7000 BCE have been found in [[Mongolia]], and the [[Natufian]] people of the [[Levant]] are known to have made knives and arrowheads from moss agate as early as 10000 BCE. Agate jewelry from [[Sumeria]] has been dated to c. 2500 BCE, and the [[Ancient Egyptians]], [[Mycenaeans]], and [[Ancient Rome|Romans]] all used agate in their jewelry.<ref name="pabian" />{{rp|pp=159–163}} Archaeological recovery at the [[Knossos]] site on [[Crete]] illustrates the role of agates in [[Bronze Age]] [[Minoan civilization|Minoan]] culture.<ref>C. Michael Hogan. 2007. [http://www.themodernantiquarian.com/site/10854/knossos.html#fieldnotes ''Knossos fieldnotes'', Modern Antiquarian] {{Webarchive|url=https://web.archive.org/web/20180711201424/http://www.themodernantiquarian.com/site/10854/knossos.html#fieldnotes |date=2018-07-11 }}</ref> The ornamental use of agate was common in [[ancient Greece]], in assorted [[jewelry]] and in the [[seal stone]]s of Greek warriors.<ref>{{cite magazine |date=7 November 2017 |title=Masterpiece of Greek Art Found in the Griffin Warrior Tomb |url=https://smithsonianmag.com/smart-news/masterpiece-greek-art-found-griffin-warrior-tomb-180967141 |magazine=[[Smithsonian (magazine)|Smithsonian]] |publisher=[[Smithsonian Institution]]}}</ref>
Agates, particularly moss agates, were first used during the [[Stone Age]] to make tools such as arrow and spear points, needles, and hide scrapers. Artifacts from as early as 7000 BCE have been found in Mongolia, and the [[Natufian]] people of the [[Levant]] are known to have made knives and arrowheads from moss agate as early as 10000 BCE. Agate jewelry from [[Sumeria]] has been dated to c. 2500 BCE, and the [[Ancient Egyptians]], [[Mycenaeans]], and [[Ancient Rome|Romans]] all used agate in their jewelry.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=159-163}} Archaeological recovery at the [[Knossos]] site on [[Crete]] illustrates the role of agates in [[Bronze Age]] [[Minoan civilization|Minoan]] culture.<ref>C. Michael Hogan. 2007. [http://www.themodernantiquarian.com/site/10854/knossos.html#fieldnotes ''Knossos fieldnotes'', Modern Antiquarian] {{Webarchive|url=https://web.archive.org/web/20180711201424/http://www.themodernantiquarian.com/site/10854/knossos.html#fieldnotes |date=2018-07-11 }}</ref> The ornamental use of agate was common in [[ancient Greece]], in assorted jewelry and in the [[seal stone]]s of Greek warriors.<ref>{{cite magazine |date=7 November 2017 |title=Masterpiece of Greek Art Found in the Griffin Warrior Tomb |url=https://smithsonianmag.com/smart-news/masterpiece-greek-art-found-griffin-warrior-tomb-180967141 |magazine=[[Smithsonian (magazine)|Smithsonian]] |publisher=[[Smithsonian Institution]]}}</ref>


[[Idar-Oberstein]] was a historically important location in Germany that made use of agate on an industrial scale, dating back to c. 1375 CE.<ref name="pabian" />{{rp|p=52}} Originally, locally found agates were used to make all types of objects for the European market, but it became a globalized business around the turn of the 20th century. Idar-Oberstein began to import large quantities of agate from Brazil, as ship's ballast. Making use of a variety of proprietary chemical processes, they produced colored beads that were sold around the globe.<ref>{{Cite web |url=http://www.farlang.com/art/2007-04-15.7721093142 |title=Background Article on Idar Oberstein |access-date=2008-11-27 |archive-date=2008-12-23 |archive-url=https://web.archive.org/web/20081223064824/http://www.farlang.com/art/2007-04-15.7721093142 |url-status=live }}</ref>
Idar-Oberstein was a historically important location in Germany that made use of agate on an industrial scale, dating back to c. 1375 CE.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=52}} Originally, locally found agates were used to make all types of objects for the European market, but it became a globalized business around the turn of the 20th century. Idar-Oberstein began to import large quantities of agate from Brazil, as ship's ballast. Making use of a variety of proprietary chemical processes, they produced colored beads that were sold around the globe.<ref>{{Cite web |url=http://www.farlang.com/art/2007-04-15.7721093142 |title=Background Article on Idar Oberstein |access-date=2008-11-27 |archive-date=2008-12-23 |archive-url=https://web.archive.org/web/20081223064824/http://www.farlang.com/art/2007-04-15.7721093142 |url-status=live }}</ref>


<gallery mode="packed" heights="130px">
<gallery mode="packed" heights="130px">
Line 189: Line 225:
File:Byzantine - The "Rubens Vase" - Walters 42562.jpg|The "[[Rubens Vase]]" ([[Byzantine Empire]]). Carved in high relief from a single piece of agate, most likely created in an imperial workshop for a Byzantine emperor.
File:Byzantine - The "Rubens Vase" - Walters 42562.jpg|The "[[Rubens Vase]]" ([[Byzantine Empire]]). Carved in high relief from a single piece of agate, most likely created in an imperial workshop for a Byzantine emperor.
File:Victorian banded agate ear rings.jpg|Victorian banded agate earrings
File:Victorian banded agate ear rings.jpg|Victorian banded agate earrings
File:Maryland Agate.jpg|Patuxent River stone from Maryland — cut and illuminated from behind as a [[nightlight]]
File:唐-玛瑙兽首杯.jpg|Agate [[drinking horn]], [[Tang dynasty]]
File:唐-玛瑙兽首杯.jpg|Agate [[drinking horn]], [[Tang dynasty]]
</gallery>
</gallery>


==Health impact==
== Treatment and processing ==
Respiratory diseases such as [[silicosis]], and a higher [[Cumulative incidence|incidence]] of [[tuberculosis]] among workers involved in the agate industry, have been studied in [[India]] and China.<ref>{{cite journal |last1=Chaudhury |first1=Nayanjeet |last2=Phatak |first2=Ajay |last3=Paliwal |first3=Rajiv |title=Co-morbidities among silicotics at Shakarpur: A follow up study |journal=Lung India |volume=29 |issue=1 |pages=6–10 |doi=10.4103/0970-2113.92348 |pmc=3276038 |pmid=22345906 |date=January 2012 |doi-access=free }}</ref><ref>{{cite journal |last1=Jiang |first1=CQ |last2=Xiao |first2=LW |last3=Lam |first3=TH |last4=Xie |first4=NW |last5=Zhu |first5=CQ |title=Accelerated silicosis in workers exposed to agate dust in Guangzhou, China. |journal=American Journal of Industrial Medicine |date=July 2001 |volume=40 |issue=1 |pages=87–91 |pmid=11439400 |doi=10.1002/ajim.1074}}</ref><ref>{{cite journal |last1=Tiwari |first1=RR |last2=Narain |first2=R |last3=Sharma |first3=YK |last4=Kumar |first4=S |title=Comparison of respiratory morbidity between present and ex-workers of quartz crushing units: Healthy workers' effect |journal=Indian Journal of Occupational and Environmental Medicine |date=September 2010 |volume=14 |issue=3 |pages=87–90 |pmid=21461160 |doi=10.4103/0019-5278.75695 |pmc=3062020 |doi-access=free }}</ref>
[[File:Agate, physical properties and origin (Plate VII) BHL48235765.jpg|thumb|Composite image of an agate slice showing natural color at the top and various artificial colors below]]
Many pale or dull agates are artificially treated to enhance their colors and make them more appealing to consumers. Chalcedony is one of the earliest stones to be artificially enhanced,<ref name="russell">{{cite web |last1=Russell |first1=Daniel |title=Historic Methods of Artificially Coloring Agates |url=https://www.mindat.org/article.php/170/Historic+Methods+of+Artificially+Coloring+Agates |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=8 September 2025 |date=13 January 2008}}</ref> with heating having been used for centuries to produce the rich red color of carnelian.<ref name="treated gem">{{cite web |title=treated gem |url=https://www.britannica.com/art/treated-gem |website=Encyclopedia Britannica |access-date=8 September 2025 |date=6 November 2016}}</ref> Many varieties of chalcedony, including agate, are relatively porous and absorb [[dye]]s well.<ref name="russell" /><ref name="treated gem" /> The classical methods<ref name="color loss">{{cite journal |last1=de Almeida Silva |first1=Rodrigo |last2=Petter |first2=Carlos Otavio |last3=Schneider |first3=Ivo André H. |title=Avaliação da perda da coloração artificial de ágatas |journal=Rem: Revista Escola de Minas |date=September 2007 |volume=60 |issue=3 |pages=477–482 |doi=10.1590/S0370-44672007000300007 |url=https://www.scielo.br/j/rem/a/ZmDd64KZTchSBcfXtGKPxvm|hdl=10183/10181 |hdl-access=free }}</ref> of staining agates were developed in the early 19th century in Idar-Oberstein, Germany. After the agates were cut and cleaned, they were soaked for several days in a particular inorganic dye or sugar solution depending on the desired color to be achieved. This was often followed by an acid bath and/or heating ("burning") to [[oxidize]] the compounds:<ref name="russell" />
 
* Blue agates were produced by using a solution of [[potassium ferricyanide]] or [[Potassium ferrocyanide|ferrocyanide]] followed by [[iron sulfate]], which forms [[Prussian blue|iron ferricyanide (Prussian blue)]].<ref name="russell" />
* Red agates were produced either by burning alone, or if not enough natural iron was present in the stones, by first soaking them in a solution of [[iron nitrate]] and then burning them to form [[iron oxide]].<ref name="russell" />
* Green agates were produced using solutions of [[nickel]] or [[chromium]] [[Salt (chemistry)|salts]] followed by burning.<ref name="russell" />
* Black agates were produced by soaking the stones in a sugar solution and then immersing them in [[sulfuric acid]] to carbonize the sugars;<ref name="russell" /> brown agates were also produced using a similar method.<ref name="russell" /><ref name="treated gem" />
* Yellow agates were relatively unpopular. They could be produced by treating iron-stained agates with [[hydrochloric acid]] in combination with burning to reduce Fe<sup>3+</sup> to Fe<sup>2+</sup>.<ref name="russell" /><ref name="treated gem" />
 
Organic [[aniline]] dyes derived from [[coal tar]] began to be used later in the 19th century,<ref name="russell" /> which allowed for the production of agates of additional colors such as pink and purple. While the colors produced by the classical methods are typically permanent, the colors produced by organic dyes can fade with exposure to light or heat.<ref name="color loss" /> Organic dyes can also only penetrate a short distance into the agate from the exposed surfaces. The practice of artificially treating agates remains popular today, and dyed Brazilian agates in particular are very common on the global market.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=157}}
 
Larger agates are often cut into halves or slices with circular diamond saws. They can then be polished with lapidary grinding, sanding, and polishing wheels of successively greater grit sizes.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=151-155}} Smaller agates and crushed agate fragments can alternatively be polished using [[Tumble finishing|rock tumblers]] or vibratory polishers. This equipment can generate large quantities of silica dust. Respiratory diseases such as [[silicosis]], and a higher [[Cumulative incidence|incidence]] of [[tuberculosis]] among workers involved in the agate industry, have been studied in India and China.<ref>{{cite journal |last1=Chaudhury |first1=Nayanjeet |last2=Phatak |first2=Ajay |last3=Paliwal |first3=Rajiv |title=Co-morbidities among silicotics at Shakarpur: A follow up study |journal=Lung India |volume=29 |issue=1 |pages=6–10 |doi=10.4103/0970-2113.92348 |pmc=3276038 |pmid=22345906 |date=January 2012 |doi-access=free }}</ref><ref>{{cite journal |last1=Jiang |first1=CQ |last2=Xiao |first2=LW |last3=Lam |first3=TH |last4=Xie |first4=NW |last5=Zhu |first5=CQ |title=Accelerated silicosis in workers exposed to agate dust in Guangzhou, China. |journal=American Journal of Industrial Medicine |date=July 2001 |volume=40 |issue=1 |pages=87–91 |pmid=11439400 |doi=10.1002/ajim.1074}}</ref><ref>{{cite journal |last1=Tiwari |first1=RR |last2=Narain |first2=R |last3=Sharma |first3=YK |last4=Kumar |first4=S |title=Comparison of respiratory morbidity between present and ex-workers of quartz crushing units: Healthy workers' effect |journal=Indian Journal of Occupational and Environmental Medicine |date=September 2010 |volume=14 |issue=3 |pages=87–90 |pmid=21461160 |doi=10.4103/0019-5278.75695 |pmc=3062020 |doi-access=free }}</ref>


==See also==
==See also==
Line 200: Line 246:
* {{annotated link|List of minerals}}
* {{annotated link|List of minerals}}
{{clear}}
{{clear}}
==Notes==
{{notelist}}


== References ==
== References ==
{{Reflist}}
{{reflist}}
 
===Sources===
*{{Cite book |last1=Pabian |first1=Roger |title=Agates: Treasures of the Earth |last2=Jackson |first2=Brian |last3=Tandy |first3=Peter |last4=Cromartie |first4=John |date=2016 |publisher=Firefly Books |isbn=978-1-77085-644-8}}
 
*{{Cite book |last1=Lynch |first1=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6}}
 
*{{cite journal |last1=Götze |first1=Jens |last2=Möckel |first2=Robert |last3=Pan |first3=Yuanming |title=Mineralogy, Geochemistry and Genesis of Agate—A Review |journal=Minerals |date=20 November 2020 |volume=10 |issue=11 |page=1037 |doi=10.3390/min10111037 |doi-access=free |bibcode=2020Mine...10.1037G}}
 
*{{cite book |last1=Moxon |first1=Terry |title=Studies on Agate |date=2009 |publisher=Terra Publications |location=Doncaster, England |isbn=978-0-9528512-1-9}}


==External links==
==External links==
{{commons category}}
{{commons category}}
* [https://web.archive.org/web/20141227143847/http://snr.unl.edu/data/geologysoils/agates/index-agates.aspx "Agates"], School of Natural Resources, University of Nebraska-Lincoln (retrieved 27 December 2014).
*[https://snr.unl.edu/data/geologysoils/agates/agateslexicon.aspx Agates Lexicon]: An alphabetical database of agate localities and variety names from the School of Natural Resources, [[University of Nebraska–Lincoln]].  


{{Silica minerals}}
{{Silica minerals}}
Line 214: Line 272:


[[Category:Agates| ]]
[[Category:Agates| ]]
[[Category:Gemstones]]
[[Category:Hardstone carving]]
[[Category:Hardstone carving]]
[[Category:Silicate minerals]]
[[Category:Symbols of Florida]]
[[Category:Symbols of Florida]]
[[Category:Provincial symbols of Nova Scotia]]
[[Category:Provincial symbols of Nova Scotia]]
[[Category:Quartz gemstones]]

Latest revision as of 16:36, 17 May 2026

For other uses, see Agate (disambiguation).

Template:Infobox mineral

Agate (/ˈæɡɪt/ AG-it) is a banded variety of fibrous chalcedony. Agate stones are characterized by alternating bands of different colored chalcedony and often contain some visible quartz crystals. They are common in nature and can be found globally in a large number of different varieties. Most agates are concentrically banded, while some have bands that form parallel lines. Certain varieties of chalcedony without bands are commonly called agate (e.g. moss agate, fire agate), but these are not true agates. Moreover, not every banded chalcedony is an agate (e.g. banded chert).

Most agates form as nodules within volcanic rock, either as amygdules or thunder eggs. Agates also form in veins within any rock type and in silicified fossils. The processes that result in agate banding are not well understood; agate has never been made synthetically.

Agate has been popular as a gemstone in jewelry for thousands of years, and today it is also popular as a collector's stone. Many duller agates sold commercially are artificially treated to enhance their color.

Etymology

The earliest known description of agate was c. 350 BCE by Theophrastus, a Greek philosopher and naturalist.[1] According to both Theophrastus and the Roman naturalist Pliny the Elder,[2] it was named for its discovery along the River Achates (Script error: The function "langx" does not exist.) in Sicily.[1][3][4] However, etymologists believe the river was probably named after the stone,[2][5] and the origin of the name is uncertain.[2] The word agate first appeared in English in the 15th century, derived from the Anglo-Norman French agathe and the Latin achātēs.[5]

Composition

Agate is composed principally of chalcedony,[6] a microscopic (microcrystalline) or submicroscopic (cryptocrystalline) form of silica. Most of the silica in chalcedony is quartz, while less than 5% to 20% is moganite, a quartz polymorph.[7][8] All forms of silica, including quartz and moganite, have a chemical composition of SiO2. They differ in that quartz has trigonal symmetry while moganite is monoclinic.[7][9] Over time, the less-stable moganite is converted to quartz;[7][10][11] as a result, no moganite has been found in agates dating from before the Silurian period.[11]

Unlike macroscopic (macrocrystalline) quartz, which is anhydrous, the chalcedony in agate normally contains small amounts of water,[7][12] between 0.5% and 3%. Most is chemically bound in the form of silanol (Si–OH), with lesser amounts of molecular H2O.[13] Silanol presence decreases with age before levelling off at about 0.4% as a side effect of the conversion of moganite to quartz; this is a potentially useful indicator of age in younger agates.[13][11] Macrocrystalline quartz is also present in most agates, particularly in the center as a solid core or geode,[14][10][15] but also occasionally in bands.[16]

Agate may also contain small amounts of opal, an amorphous form of silica[12] containing up to 10% molecular water.[13] Most is in the form of opal-CT, which consists of nanometer-sized crystals of the silica polymorphs cristobalite and tridymite and is therefore not truly amorphous. Fully amorphous opal-A is rare in agates. Like moganite, both varieties of opal are metastable and are eventually converted to quartz.[10]

Impurities

Agate often contains mineral inclusions or trace elements incorporated into the crystal structure. The most common trace elements present in agate are aluminum, calcium, potassium, sodium, and iron.[17] Iron is responsible for producing the color of most agate bands;[18] red and yellow bands are often colored by microscopic spheres of the iron oxides hematite and goethite, respectively.[17][18] Manganese, chromium, and nickel may also be present in some colored bands.[17] Some agates contain very small traces of uranium in the form of uranyl ions, causing them to fluoresce green under short-wave ultraviolet light.[19][20] The most common mineral inclusions in agate are calcite and zeolites.[21]

Structure

Fibers

File:Agate Rio Grande do Sul Caillois Donation MNHN n18.jpg
When backlit, thin slices of agate can occasionally show their fibrous structure as lines running perpendicular to the bands (top)[6]

The chalcedony in agate is said to be fibrous, forming chain-like "fibers" of intergrown crystals.[6][10] These fibers are twisted along their length,[22][10][12] giving them a helical shape.[23] Individual fibers were determined by X-ray diffraction to be 0.1-1.0 µm in diameter and up to several millimeters long.[10] There are two different types of chalcedony fibers: length-slow (also known as quartzine) and length-fast.[7][10] Agate primarily contains length-fast chalcedony fibers, consisting of crystals stacked perpendicular to the c-axis (side to side). Agate rarely contains quartzine, consisting of quartz crystals stacked parallel to the c-axis (tip to tip).[6][7][10] Quartzine only occurs in the outer layer of agates or as thin intergrowths between layers of length-fast chalcedony fibers and macrocrystalline quartz.[10]

Banding

File:Agate Rio Grande do Sul Caillois Donation MNHN n01.jpg
Agate exhibiting wall banding (top) and level banding (bottom)

Agates are broadly separated into two categories based on the type of banding they exhibit:[6][24][25]

  1. Wall banding[lower-alpha 1] occurs when agate bands roughly follow the shape of the cavity they formed in. In wall-banded agates, chalcedony fibers grow radially from the cavity walls inward, perpendicular to the direction of the bands.[6][27]
  2. Level banding[lower-alpha 2] occurs when agate bands form in straight, parallel lines. In level-banded agates, chalcedony precipitates out of solution in the direction of gravity, resulting in horizontal layers of microscopic chalcedony spherulites.[6][25]

Level banding is less common and usually occurs together with wall banding.[6] Wall-banded agate is more fibrous than level-banded agate, which is more granular in texture. Consequently, wall-banded agate is denser and more compact.[25]

Formation

File:Agate- & quartz-lined geode 5 (32375570960).jpg
Agate geode slice with macrocrystalline quartz and a hollow center
File:Agate Rio Grande do Sul Caillois Donation MNHN n26.jpg
Agate with level banding in the center surrounded by layers of wall banding

Geologists generally understand the early stages of agate formation, but the specific processes that result in band development are widely debated. Since they form in cavities within host rock, agate formation cannot be directly observed,[14] and agate banding has never been successfully replicated in the lab.[23][28]

Nodular agate

Agates are most commonly found as nodules within volcanic rocks.[29][30] In mafic rocks such as basalt and andesite, they form inside cavities called vesicles (amygdaloids[lower-alpha 3] when filled),[32][33] gas bubbles that were trapped inside the lava when it cooled.[14][32][31] Since mafic lavas are poor in free silica, there are multiple theories of where the silica originates from, including micro-shards of silica glass from volcanic ash or tuff deposits and decomposing plant or animal matter.[12] In felsic rocks such as rhyolite and rhyolitic tuff, agates form in spherulite-filled cavities called lithophysae, commonly called thunder eggs when filled with agate.[30][34]

The cavities are filled with hot, silica-rich water from the surrounding environment, forming a silica gel. This gel crystallizes through a complex process to form agates.[35] Agates are much harder than the rocks they form in; some varieties (e.g. Lake Superior agates) are frequently found detached from their host rock.[36]

In mafic rocks, the vesicle walls are often coated with thin layers of celadonite or chlorite,[14][15] soft, green phyllosilicate minerals that form from the reaction of hot, silica-rich water with the rock.[14] This coating provides a rough surface for the chalcedony fibers to form on, initially as radial spherulites. The rough surface also causes agate husks to have a pitted appearance once the coating has been weathered away or removed.[37] Sometimes, the spherulites grow around mineral inclusions, resulting in eyes, tubes, and sagenitic agates.[6]

The first layer of spherulitic chalcedony is typically clear, followed by successive growth bands of chalcedony alternated with chemically precipitated color bands, primarily iron oxides.[38] The center is often macrocrystalline quartz,[14] which can also occur in bands and possibly forms when there is not enough chemically bound water in the silica gel to promote chalcedony polymerization.[15] When the silica concentration of the gel is too low, a hollow center forms, called an agate geode. In geodes, quartz forms crystals around the cavity, with the apex of each crystal pointing towards the center. Occasionally, quartz in agates may be colored, occurring in varieties such as amethyst or smoky quartz.[16]

Level bands usually form at the base of the vesicle or in the center when the gel stops adhering to the vesicle walls. This is probably due to a decrease in bound water in the gel.[25]

Less commonly, agates can form as nodules within sedimentary rocks such as limestone, dolomite or tuff. These agates form when silica replaces another mineral, or silica-rich water fills cavities left by decomposed plant or animal matter.[39]

Vein agate

Agates can also form within rock fissures, called veins.[39] Vein agates form in a manner similar to nodular agates (see above),[38] and they include lace agates such as blue lace agate and crazy lace agate. Veins may form in either volcanic rock or sedimentary rock.[40]

Fossil agate

Fossil agates form when silica replaces the original composition of an organic material.[41] This process is called silicification, a form of petrification. Examples include petrified wood,[42] agatized coral,[43] and Turritella agate (Elimia tenera).[44] Although these fossils are often referred to as being "agatized", they are only true agates if they contain bands.[6]

Variations

Besides the two fundamental types of banding (wall and level), agates can exhibit a wide variety of band patterns, internal structures, and optical effects.[6][45]

Band patterns

  • Fortification agates are any wall-banded agates with sharp, well-defined bands.[46][47] They are named for their appearance, which resembles the walls of a fort. Some authors do not make a distinction between fortification agates and other wall-banded agates.[26]
  • Lace agates are vein agates that exhibit a lace-like pattern of bands with many swirls, eyes, bends, and zigzags.[48]
  • Faulted agates have bands that were broken and slightly shifted by rock movement and then re-cemented together by chalcedony. They have the appearance of rock layers with fault lines running through them.[49][50]
  • Brecciated agates also have bands that were broken apart and re-cemented with chalcedony, but they consist of disjointed band fragments at random angles.[51][50] They are a form of breccia, which is a textural term for any rock composed of angular fragments.[51][52]
  • Eye agates have one or more circular, concentric rings on their surface.[53] These "eyes" are actually hemispheres that form on the husk of the agate and extend inward like a bowl.[54]
  • Onyx is the traditional name for agates with level banding, although the formal definition of the term onyx refers to color pattern, not the shape of the bands.[55] Accordingly, the name onyx is also used for wall-banded agates. Onyx is frequently misused as a name for banded calcite. The name originates from the Greek word for the human nail, which has parallel ridges.[56] Typically, onyx bands alternate between black and white or other light and dark colors.
  • Sardonyx is a variety of onyx with red-to-brown bands alternated with either white or black bands.[57]

Internal structures

  • Sagenitic agates, or sagenites, have acicular (needle-shaped) inclusions of another mineral, usually anhydrite, aragonite, goethite, rutile, or a zeolite. Chalcedony often forms tubes around these crystals and may eventually replace the original mineral, resulting in a pseudomorph.[58] The term "sagenite" was originally a name for a type of rutile, and later rutilated quartz. It has since been used to describe any quartz variety with acicular inclusions of any mineral.[59]
  • Tube agates contain tunnel-like structures that extend all the way through the agate.[60] These "tubes" may sometimes be banded or hollow, or both. Tube agates form when chalcedony grew around sagenitic inclusions embedded within the agate, forming stalactitic structures. Visible "eyes" can also appear on the surface of tube agates if a cut is made (or the agate is weathered) perpendicular to the stalactitic structure.[61]
  • Dendritic agates have dark-colored, fern-patterned inclusions (dendrites) that form on the surface of agates or in the spaces between bands.[62][63][64] They are composed of manganese oxides or iron oxides.[63]
  • Moss agates exhibit a moss-like inclusions and are usually green or brown in color.[65][66] They form when dendritic structures on the surface of an agate are pushed inward with the silica gel during their formation. Moss agate was once believed to be petrified moss,[65] until it was discovered the moss-like formations are actually composed of celadonite, hornblende, or a chlorite mineral.[65][66]
  • Plume agates are a type of moss agate, but the dendritic "plumes" form tree-like structures within the agate. They are often bright red (from inclusions of hematite) or bright yellow (from inclusions of goethite).[65][67] Both moss and plume agates usually lack any form of banding, and therefore are not true agates according to the mineralogical definition.[6][68]
  • Enhydro agates, or enhydros, form when bubbles of liquid water become trapped within an agate (or chalcedony) nodule or geode, often long after its formation.[69][70]

Optical effects

  • Iris agates have bands fine enough that when thinly sliced, they cause transmitted light to be diffracted into its spectral colors.[71][72] This "iris effect" usually occurs in colorless agates, but it can also occur in brightly colored ones.[71]
  • Parallax banding, also called shadow banding, can occur when an agate has very fine, alternating bands of transparent and opaque chalcedony. A shadow is cast onto the opaque bands which appears to move when the agate is viewed from different angles.[73]

Local varieties

Agates are very common, and they have been found on every continent,[74] including Antarctica.[75] In addition to the structural varieties detailed in the previous section, numerous geological, local, and trade names are used to describe agates from different localities.[76] Below is a table of agate varieties from different regions of the world.Template:Sticky header

Local varieties of agate
Name Locality Description Type Geologic environment Age Photo(s) Template:Ref.
(Unnamed agate) Bellingshausen Station, King George Island, Antarctica White and clear bands Nodular agate File:Agate (Tertiary; Agate Bay, King George Island, South Shetland Islands) 1.jpg
Agate from King George Island, Antarctica		 [75]
Blue lace agate Primarily Namibia; also Malawi, Kenya, Turkey, Georgia, and Zambia Pale blue and white lace agate Vein agate Volcanic rock (dolomite associated with dolerite) Jurassic period File:Blue Lace Agate.jpg
Blue lace agate from Ysterputz Mine, Namibia		 [77][20]
Blue Bed (Pony Butte) thunder egg Richardson Ranch (formerly Priday Ranch), northeast of Madras, Oregon, United States Blue and white banding with dark brown shell, frequently level-banded Nodular agate (thunder egg) Volcanic rock (John Day Formation, rhyolitic volcanic ash) Miocene epoch File:Thunder Egg Agate (Priday Blue Bed, John Day Formation, Miocene; near Madras, Oregon, USA) 3 (33992544563).jpg
Blue Bed thunder egg from near Madras, Oregon		 [78]
Boley agate Central Oklahoma, United States White fortification and eye banding with clasts of brecciated chert Vein agate Sedimentary rock (Boley conglomerate layer, Vamoosa formation) Virgilian series Template:Image requested inline [79]
Botswana agate Botswana Typically 2.5–5 cm (0.98–1.97 in) in diameter, with contrasting bands of purple, pink, black, grey, and white Nodular agate Volcanic rock (Karoo Series, basalt) Permian period File:Agat - Bobonong, Botswana.jpg
Botswana agate		 [80]
Brazilian agate Rio Grande do Sul and other southeastern states, Brazil Often large, up to 0.9 m (3.0 ft) in diameter and over 120 kg (260 lb), commonly pale yellow, gray, or colorless (usually sold artificially dyed), are more colorful or contain structural features Nodular agate Volcanic rock (decomposed volcanic ash and basalt) Late Permian period File:Brazilian agate section (detail) (cropped).JPG
Natural Brazilian agate

File:5agat, Brazylia.jpg
Dyed Brazilian agate		 [81]
Condor agate Mendoza, Argentina Bright red and yellow fortification banding, may contain mossy or sagenitic inclusions Nodular agate File:Argentina001 (cropped).jpg
Condor agate		 [82]
Coldwater agate (Lake Michigan cloud agate) Great Lakes Region, United States Banded lines of grey and white chalcedony Nodular agate Sedimentary rock (marine limestone and dolomite) Template:Image requested inline [83]
Crater agate Patagonia, Argentina Typically hollow, black with red bands near the center Nodular agate Volcanic rock (rhyolite) Jurassic period Template:Image requested inline [84]
Crazy lace agate Mexico Brightly colored lace agate, typically white and red, sometimes yellow and grey Vein agate Sedimentary rock Late Cretaceous period File:Crazy Lace Agate - Macro Panorama.jpg
Crazy lace agate		 [85][86]
Crowley's Ridge agate Crowley's Ridge, Arkansas and Missouri, United States Pale yellow to tan fortification banding, rarely pink to red (purportedly due to heating in wildfires). Also found along the Mississippi River basin in Mississippi and Louisiana. Nodular agate Sedimentary rock Paleozoic era (deposited in Pliocene gravel) [87][88][89]
Dugway geode Utah, United States Light grey to blue, often contain hollow cavities lined with drusy quartz Nodular agate (thunder egg) File:Dugway Geode (Juab County, Utah, USA) 2 (34581522545).jpg
Dugway geode from Utah		 [90]
Fairburn agate South Dakota and Nebraska, United States Red fortification banding Nodular agate Sedimentary rock (marine carbonate sediments) Pennsylvanian period File:Fairburn Agate (ultimately derived from the Minnelusa Formation, Pennsylvanian-Permian; collected east of the Black Hills, western South Dakota, USA) 26 (32406082220).jpg
Fairburn agate from western South Dakota		 [91][92]
German agate Near Idar-Oberstein, Germany Often red or pink, sometimes other colors Nodular agate Volcanic rock Permian period File:Agate-MCG 91225-P4150832-white.jpg
German agate from Idar-Oberstein		 [93]
Holley (Holly) blue agate Near Holley, Oregon, United States Lavender to blue Nodular agate File:Holley Blue Agate (Linn County, Oregon, USA) 9.jpg
Holley blue agate from Oregon		 [94]
Laguna agate Ojo Laguna, Chihuahua, Mexico Vibrant bands in shades of red, orange, pink, or purple, often exhibit parallax or shadow banding, inclusions common Nodular agate Volcanic rock (andesite) Tertiary period File:LagunaAgateFromMexico (cropped).jpg
Laguna agate		 [95][96]
Lake Superior agate Near Lake Superior, United States and Canada Bands in shades of red, orange, yellow, brown, white, and grey, level banding and various structural features common Nodular agate Volcanic rock (basalt) Late Precambrian (can be found in Pleistocene glacial deposits) File:Agate nodule ("Lake Superior Agate") (floor of Lake Superior, offshore Keweenaw Peninsula, Michigan USA) 2 (33741645898).jpg
Rough Lake Superior agate from Keweenaw Peninsula, Michigan		 [97][98][99]
Lysite agate Lysite Mountain, Fremont County, Wyoming, United States Colorful bands with plumes and moss Vein agate Sedimentary rock (marine origin) File:Lysite Agate Wyoming.jpg
Lysite agate from Wyoming		 [100]
Malawi agate Malawi Typically bright red or orange with contrasting white bands, some are pink and blue Nodular agate Volcanic rock Permian period File:Malawi Agate (Malawi, southeastern Africa) (32734668126).jpg
Malawi agate		 [101]
Potato stone (Pot stone) Bristol and Somerset, England, United Kingdom Irregularly-shaped, reddish, banded agate nodules, typically surrounding a hollow cavity lined with macroscopic quartz, but sometimes completely filled Nodular agate Sedimentary rock (dolomitic conglomerate and marl) Triassic period File:Quartz-agate (12250382174).jpg
Potato stone from England		 [102]
Puma agate Andes, Patagonia, Argentina Agatized coral Fossil agate Sedimentary rock (marine) Template:Image requested inline [84]
Queensland agate Queensland, Australia Often green or yellow-green (colors that are rarely found in other regions), frequently level-banded Nodular agate Volcanic rock (basaltic lava flows) Late Permian period File:Agat ze strefą krustalną - Agate Creek, Queensland, Australia.jpg
Queensland agate with level banding		 [103]
Scottish agate Stonehaven to just south of Ayr, near Oban, and surrounding the Cheviot Hills, Scotland, United Kingdom Various colored bands Nodular agate Volcanic rock (andesite) Early Devonian period File:Agate detail, Scotland 007 (cropped).jpg
Close-up of a Scottish agate from Ayrshire		 [104]
Small Isles agate Islands off the west coast of Scotland, United Kingdom Various colored bands Nodular agate Volcanic rock (basalt) Tertiary period Template:Image requested inline [104]
Sweetwater agate Near Sweetwater River, Wyoming, United States Small moss agates with brown or black dendrites, fluorescent under UV light Nodular agate Sedimentary rock (sandstone) Miocene epoch Template:Image requested inline [100]
Turritella agate Wyoming, United States Brown fossil agate with the elongated spiral shells of an extinct freshwater snail (Elimia tenera) Fossil agate Sedimentary rock (Green River Formation) Eocene epoch File:Elimia fossils Wyoming.jpg
Turritella agate (Elimia tenera)		 [44]

Uses

Agate is frequently used as a gemstone in jewelry such as pins, brooches, necklaces, earrings, and bracelets. Agates have also historically been used in the art of hardstone carving to make knives, inkstands, seals, marbles, and other objects. Today, they are widely used to make beads, decorative displays, carvings, and cabochons, as well as face-polished and tumble-polished specimens of varying size and origin. Agate collecting is a popular hobby, and agate specimens can be found in numerous gift shops, museums, galleries, and private collections.[105]

Industrial uses of agate exploit its hardness, ability to retain a highly polished surface finish and resistance to chemical attack. Historically, it was used to make bearings for highly accurate laboratory balances and mortars and pestles to crush and mix chemicals. During the Second World War, black agate beads mined from Queensland, Australia were used in the turn and bank indicators of military aircraft.[106]

Agates, particularly moss agates, were first used during the Stone Age to make tools such as arrow and spear points, needles, and hide scrapers. Artifacts from as early as 7000 BCE have been found in Mongolia, and the Natufian people of the Levant are known to have made knives and arrowheads from moss agate as early as 10000 BCE. Agate jewelry from Sumeria has been dated to c. 2500 BCE, and the Ancient Egyptians, Mycenaeans, and Romans all used agate in their jewelry.[107] Archaeological recovery at the Knossos site on Crete illustrates the role of agates in Bronze Age Minoan culture.[108] The ornamental use of agate was common in ancient Greece, in assorted jewelry and in the seal stones of Greek warriors.[109]

Idar-Oberstein was a historically important location in Germany that made use of agate on an industrial scale, dating back to c. 1375 CE.[110] Originally, locally found agates were used to make all types of objects for the European market, but it became a globalized business around the turn of the 20th century. Idar-Oberstein began to import large quantities of agate from Brazil, as ship's ballast. Making use of a variety of proprietary chemical processes, they produced colored beads that were sold around the globe.[111]

Treatment and processing

File:Agate, physical properties and origin (Plate VII) BHL48235765.jpg
Composite image of an agate slice showing natural color at the top and various artificial colors below

Many pale or dull agates are artificially treated to enhance their colors and make them more appealing to consumers. Chalcedony is one of the earliest stones to be artificially enhanced,[112] with heating having been used for centuries to produce the rich red color of carnelian.[113] Many varieties of chalcedony, including agate, are relatively porous and absorb dyes well.[112][113] The classical methods[114] of staining agates were developed in the early 19th century in Idar-Oberstein, Germany. After the agates were cut and cleaned, they were soaked for several days in a particular inorganic dye or sugar solution depending on the desired color to be achieved. This was often followed by an acid bath and/or heating ("burning") to oxidize the compounds:[112]

Organic aniline dyes derived from coal tar began to be used later in the 19th century,[112] which allowed for the production of agates of additional colors such as pink and purple. While the colors produced by the classical methods are typically permanent, the colors produced by organic dyes can fade with exposure to light or heat.[114] Organic dyes can also only penetrate a short distance into the agate from the exposed surfaces. The practice of artificially treating agates remains popular today, and dyed Brazilian agates in particular are very common on the global market.[115]

Larger agates are often cut into halves or slices with circular diamond saws. They can then be polished with lapidary grinding, sanding, and polishing wheels of successively greater grit sizes.[116] Smaller agates and crushed agate fragments can alternatively be polished using rock tumblers or vibratory polishers. This equipment can generate large quantities of silica dust. Respiratory diseases such as silicosis, and a higher incidence of tuberculosis among workers involved in the agate industry, have been studied in India and China.[117][118][119]

See also

Notes

  1. ↑ Also called concentric banding,[6][26] adhesional banding, or fortification banding[26]
  2. ↑ Also called water-level banding, gravitational banding, parallel banding,[24] horizontal banding, or Uruguay-type banding[6][24]
  3. ↑ Amygdaloid means "almond-shaped," but they may also be round, irregular, flat, or bun-shaped.[31]

References

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  2. ↑ 2.0 2.1 2.2 "Achate, N. (1), Etymology." Oxford English Dictionary. Oxford University Press. Retrieved 6 May 2026. doi:10.1093/OED/6418139009
  3. ↑ Theophrastus; Caley, Earle R.; Richards, John F.C (1956). Theophrastus. On Stones. Columbus, Ohio: The Ohio State University. p. 52. Retrieved 6 May 2026. The achates is also a beautiful stone; it comes from the river Achates in Sicily and is sold at a high price.
  4. ↑ Pliny the Elder; Bostock, John; Riley, H. T. "Book 37, Chapter 54". Naturalis historia. Perseus Digital Library. Retrieved 6 May 2026. Achates was a stone formerly in high esteem, but now held in none. It was first found in Sicily, near a river of that name; but has since been discovered in numerous other localities.
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  85. ↑ Atkinson, Bill; Ackerman, Diane (2004). Within the Stone: Photography. BrownTrout Publishers. ISBN 978-0-7631-8189-5. Archived from the original on 2023-08-26. Retrieved 2020-10-29.
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  87. ↑ "Crowley's Ridge Agates". Dwarves' Earth Treasures. Retrieved 17 February 2026.
  88. ↑ "Agate from Crowley's Ridge (Crowleys Ridge), Malden, Dunklin County, Missouri, USA". mindat.org. Hudson Institute of Mineralogy. Retrieved 17 February 2026.
  89. ↑ Guccione, M. J.; Prior, W. L.; Rutledge, E. M. (1986). "The Tertiary and Quaternary Geology of Crowley's Ridge: A Guidebook". Office of the State Geologist. Arkansas Geological Commission. Retrieved 17 February 2026.
  90. ↑ Pabian et al. 2016, p. 92.
  91. ↑ "Fairburn Agate". mindat.org. Hudson Institute of Mineralogy. Retrieved 16 February 2025.
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  95. ↑ "Laguna Agate". www.mindat.org. Retrieved 2025-02-16.
  96. ↑ Pabian et al. 2016, pp. 114–115.
  97. ↑ "Lake Superior Agate". mindat.org. Hudson Institute of Mineralogy. Retrieved 16 February 2025.
  98. ↑ Lynch & Lynch 2012.
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  108. ↑ C. Michael Hogan. 2007. Knossos fieldnotes, Modern Antiquarian Archived 2018-07-11 at the Wayback Machine
  109. ↑ "Masterpiece of Greek Art Found in the Griffin Warrior Tomb". Smithsonian. Smithsonian Institution. 7 November 2017.
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  111. ↑ "Background Article on Idar Oberstein". Archived from the original on 2008-12-23. Retrieved 2008-11-27.
  112. ↑ 112.00 112.01 112.02 112.03 112.04 112.05 112.06 112.07 112.08 112.09 Russell, Daniel (13 January 2008). "Historic Methods of Artificially Coloring Agates". mindat.org. Hudson Institute of Mineralogy. Retrieved 8 September 2025.
  113. ↑ 113.0 113.1 113.2 113.3 "treated gem". Encyclopedia Britannica. 6 November 2016. Retrieved 8 September 2025.
  114. ↑ 114.0 114.1 de Almeida Silva, Rodrigo; Petter, Carlos Otavio; Schneider, Ivo André H. (September 2007). "Avaliação da perda da coloração artificial de ágatas". Rem: Revista Escola de Minas. 60 (3): 477–482. doi:10.1590/S0370-44672007000300007. hdl:10183/10181.
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  116. ↑ Pabian et al. 2016, pp. 151–155.
  117. ↑ Chaudhury, Nayanjeet; Phatak, Ajay; Paliwal, Rajiv (January 2012). "Co-morbidities among silicotics at Shakarpur: A follow up study". Lung India. 29 (1): 6–10. doi:10.4103/0970-2113.92348. PMC 3276038. PMID 22345906.
  118. ↑ Jiang, CQ; Xiao, LW; Lam, TH; Xie, NW; Zhu, CQ (July 2001). "Accelerated silicosis in workers exposed to agate dust in Guangzhou, China". American Journal of Industrial Medicine. 40 (1): 87–91. doi:10.1002/ajim.1074. PMID 11439400.
  119. ↑ Tiwari, RR; Narain, R; Sharma, YK; Kumar, S (September 2010). "Comparison of respiratory morbidity between present and ex-workers of quartz crushing units: Healthy workers' effect". Indian Journal of Occupational and Environmental Medicine. 14 (3): 87–90. doi:10.4103/0019-5278.75695. PMC 3062020. PMID 21461160.

Sources

  • Pabian, Roger; Jackson, Brian; Tandy, Peter; Cromartie, John (2016). Agates: Treasures of the Earth. Firefly Books. ISBN 978-1-77085-644-8.
  • Lynch, Dan R.; Lynch, Bob (2012). Lake Superior Agates Field Guide. Adventure Publications. ISBN 978-1-59193-282-6.

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