Editing Agate (section)

==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}}</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" />