Editing Agate

{{short description|Banded variety of chalcedony}}
{{other uses}}
{{Infobox mineral
| name = Agate
| category = [[Tectosilicate]] [[minerals]], [[quartz]] [[Mineral group|group]], [[chalcedony]] [[Mineral variety|variety]]
| boxwidth =
| boxbgcolor =#bb361c
| boxtextcolor = #fff
| image = Mexican Crazy Lace Agate - World's Best.jpg
| imagesize = 260px
| 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
| 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 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/cm³ 
| 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 [[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.

==Etymology ==
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==
[[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.

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>

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}}
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.

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" />

==Varieties (by 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" />

=== Wall-banded agates ===
''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>

''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}}

''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>

''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}}

''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>

''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}}

''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>

=== Level-banded agates ===
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>

<gallery mode="packed" heights="130px">
Agate 6 (32375570980).jpg|Agate with both wall banding (top) and level banding (bottom)
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 eye agates
Detail, Dendritic agate (cropped).jpg|Dendritic agate from India
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>

== Varieties (by locality) ==
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.

=== 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 ===
* 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 ===
* India has produced agates since as early as the 11th century. These include carnelian agates, moss agates, and dendritic agates.
* 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.
* 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 ===
* ''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 ===
* 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.
* [[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. 
* [[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. 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.
* ''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>

=== North America ===
* ''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 ===
* ''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">
Brazilian agate section (detail).JPG|Natural Brazilian agate
5agat, Brazylia.jpg|Dyed Brazilian agate
Argentina001.jpg|Condor agate
Achat 1.jpg|Agate from Uruguay
</gallery>

== 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}}

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}}

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>

[[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>

<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|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: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]]
</gallery>

==Health impact==
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==
{{Portal|Geology|Earth sciences|Minerals}}
* {{annotated link|List of minerals}}
{{clear}}
== References ==
{{Reflist}}

==External links==
{{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).

{{Silica minerals}}
{{Gemstones}}
{{Jewellery}}
{{Authority control}}

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