{{Short description|Banded variety of chalcedony}} {{Other uses}} {{Good article}} {{Infobox mineral | name = Agate | category = Tectosilicate minerals | group = Quartz group | IMAstatus = Variety of quartz (chalcedony) | boxwidth = | boxbgcolor =#bb361c | boxtextcolor = #fff | image = Malawi Agate (Malawi, southeastern Africa) (32734668126).jpg | imagesize = | caption = Polished agate nodule from Malawi | formula = SiO<sub>2</sub> (silicon dioxide) | molweight = | color = Often multicolored; commonly colorless, pale blue to black, red to orange, yellow, white, brown, pink, purple; rarely green | habit = Cryptocrystalline silica | system = Trigonal (quartz) or monoclinic (moganite) | twinning = | cleavage = None | fracture = Conchoidal | mohs = 6.5–7 | luster = Waxy, vitreous when polished | refractive = 1.530-1.543 | opticalprop = Uniaxial (+) | birefringence = Up to 0.004 | dispersion= None | pleochroism = Absent | streak = White | gravity = 2.60–2.64 | density = 2.6 g/cm<sup>3</sup> | melt = | tenacity = Brittle | fusibility = | diagnostic = | solubility = | diaphaneity = Transparent to opaque (usually translucent) | other = | references= <ref name="mindat" /><ref>{{cite web |title=Agate |url=https://www.gemdat.org/gem-51.html |website=gemdat.org |access-date=9 March 2025}}</ref><ref>{{cite web |title=Agate Value, Price, and Jewelry Information |url=https://www.gemsociety.org/article/agate-gem-information/ |website=gemsociety.org |publisher=International Gem Society |access-date=9 March 2025}}</ref> }}

'''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).

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 {{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 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 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>

==Composition== Agate is composed principally of chalcedony,<ref name="mindat" /> 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.<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" />

Unlike 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 (Si–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}}

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 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}}

===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.{{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 zeolites.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=17,19}}

==Structure== ===Fibers=== 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 helical shape.<ref name="PSU" /> Individual fibers were determined by X-ray diffraction to be 0.1-1.0 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}}

===Banding=== 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}}

#''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 spherulites.<ref name="mindat" />{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=19}}

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}}

==Formation== thumb|Agate geode slice with macrocrystalline quartz and a hollow center thumb|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,{{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>

===Nodular agate=== Agates are most commonly found as nodules within 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 ''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 ''thunder eggs'' when filled with agate.{{sfn|Götze|Möckel|Pan|2020|loc=3}}{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=38,49}}

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 agates) are frequently found detached from their host rock.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=84}}

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 spherulites. 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" />

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 varieties such as amethyst or smoky quartz.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=17}}

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}}

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.{{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.{{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 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 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 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 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 Onyx Mainzer Becken.jpg|Onyx from Germany </gallery>

===Internal structures=== *{{anchor|Sagenitic agate}}''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.{{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>

*{{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 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}}

*{{anchor|Dendritic agate}}''Dendritic agates'' have dark-colored, fern-patterned inclusions (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 oxides or iron oxides.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=25}} *''Moss agates'' 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 mineral.{{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=32-33}}<ref name="moss" /> *''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>

*''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>

<gallery mode="packed"> Agate Rio Grande do Sul Caillois Donation MNHN n24.jpg|Sagenitic agate from Brazil with numerous needle-like inclusions Detail, Dendritic agate (cropped).jpg|Close-up of a dendrite in an agate from India Four moss agate cabochons.jpg|Moss agate cabochons </gallery>

===Optical effects=== *{{anchor|Iris agate}}''Iris agates'' have bands fine enough that when thinly sliced, they cause transmitted light to be diffracted into its 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}}

*''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}}

<gallery mode="packed"> 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 </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 || || || 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, and Zambia || Pale blue and white lace agate || Vein agate || Volcanic rock (dolomite associated with dolerite) || Jurassic period || 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 (thunder egg) || Volcanic rock (John Day Formation, rhyolitic volcanic ash) || Miocene epoch || 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 || 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 || Volcanic rock (Karoo Series, basalt) || Permian period || 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 || Volcanic rock (decomposed volcanic ash and basalt) || Late Permian period || 130px<br />Natural Brazilian agate <br><br> 130px<br />Dyed Brazilian agate || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=122-126}} |- | Condor agate || Mendoza, Argentina || Bright red and yellow fortification banding, may contain mossy or sagenitic inclusions || Nodular agate || || || 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, United States|| Banded lines of grey and white chalcedony || 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 || 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 || Sedimentary rock || Late Cretaceous period || 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 || 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 (thunder egg) || || || 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 || Sedimentary rock (marine carbonate sediments) || Pennsylvanian period || 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 || Volcanic rock || Permian period || 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 || || || 130px<br />Holley blue agate from Oregon || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=103}} |- | {{anchor|Laguna agate}}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 || 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 || Volcanic rock (basalt) || Late Precambrian (can be found in Pleistocene glacial deposits) || 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 || Sedimentary rock (marine origin) || || 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 || Volcanic rock || Permian period || 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 || Sedimentary rock (dolomitic conglomerate and marl) || Triassic period || 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 || 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 || Volcanic rock (basaltic lava flows) || Late Permian period || 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 || Volcanic rock (andesite) || Early Devonian period || 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 || Volcanic rock (basalt) || Tertiary period || {{Image requested inline}} || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|pp=58-61}} |- | {{anchor|Sweetwater agate}}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 || {{Image requested inline}} || {{sfn|Pabian|Jackson|Tandy|Cromartie|2016|p=79}} |- | 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 || 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 == 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.{{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 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.{{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. 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.{{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 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 stones 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 |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.{{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"> File:Tumbled agate and jasper.jpg|A {{convert|15|lb|kg|abbr=on|order=flip}} barrel full of tumble-polished agate and jasper File:Zegelring in goud met intaglio met portret van Commodus in nicolo, 180 tot 200 NC, vindplaats- Tongeren, de Schaetzengaarde 22, 1998, losse vondst (mogelijk goudschat), collectie Gallo-Romeins Museum Tongeren, GRM 1892.jpg|Gold Roman signet ring with portrait of emperor Commodus in niccolo agate, 180-200 CE, found in Tongeren, Gallo-Roman Museum (Tongeren) 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:唐-玛瑙兽首杯.jpg|Agate drinking horn, Tang dynasty </gallery>

== Treatment and processing == 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 dyes 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 ferrocyanide followed by iron sulfate, which forms 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 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 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.<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== {{Portal|Geology|Earth sciences|Minerals}} * {{annotated link|List of minerals}} {{clear}}

==Notes== {{notelist}}

== References == {{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== {{commons category}} *[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}} {{Gemstones}} {{Jewellery}} {{Authority control}}

Category:Agates Category:Hardstone carving Category:Symbols of Florida Category:Provincial symbols of Nova Scotia Category:Quartz gemstones