chalcedony

Chalcedony occurs in a wide range of varieties. Many semi-precious gemstones are in fact forms of chalcedony. The more notable varieties of chalcedony are as follows:

File:Quartz - Agateplate, redbrown-white.jpg

Agate is a fibrous, banded variety of chalcedony that occurs in a variety of colors and patterns.{{cite web |title=Agate |url=https://www.mindat.org/min-51.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=10 February 2025}} Iris agate shows exceptional iridescence when light (especially pinpointed light) is shone through the stone. Landscape agate is chalcedony with a number of different mineral impurities making the stone resemble landscapes.{{Cite web |url=http://www.cibjo.org/index.php?option=com_content&view=article&id=55&Itemid=32 |title=Retailers' Reference Guide: Diamonds, Cemstones, Pearls and Precious Metals |author=CIBJO member laboratories |date=May 2009 |publisher=CIBJO (The World Jewellery Federation, international federation of all national trade organizations and gemological laboratories) |location=Bern, Switzerland }}

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File:carnelian sard (mineral specimen).jpg

Carnelian (also spelled cornelian) is a clear-to-translucent reddish-brown variety of chalcedony. Its hue may vary from a pale orange to an intense almost-black coloration. Similar to carnelian is sard, which is brown rather than red.

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Chrysoprase (also spelled chrysophrase) is a green variety of chalcedony, which has been colored by nickel oxide. (The darker varieties of chrysoprase are also referred to as prase. However, the term prase is also used to describe green quartz and to a certain extent is a color-descriptor, rather than a rigorously defined mineral variety.)

Blue-colored chalcedony is sometimes referred to as "blue chrysoprase" if the color is sufficiently rich, though it derives its color from the presence of copper and is largely unrelated to nickel-bearing chrysoprase.

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Fire agate is a variety of chalcedony with inclusions of goethite or limonite causing an iridescent effect.{{Cite web|title=Fire Agate|url=https://www.mindat.org/min-7601.html|access-date=2021-04-11|website=www.mindat.org}} It can display a wide range of iridescent colors including red, orange, yellow, green, blue, and purple.{{Cite web|last=Page|first=Geology|date=2020-01-13|title=Fire Agate : What is fire agate? How fire agate is formed?|url=http://www.geologypage.com/2020/01/fire-agate.html|access-date=2021-04-11|website=Geology Page|language=en-US}} {{Clear}}

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Heliotrope is a green variety of chalcedony, containing red inclusions of iron oxide that resemble drops of blood, giving heliotrope its alternative name of bloodstone. In a similar variety known as plasma, the spots are yellow instead.

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Moss agate contains green filament-like inclusions, giving it the superficial appearance of moss or blue cheese. There is also tree agate which is similar to moss agate except it is solid white with green filaments whereas moss agate usually has a transparent background, so the "moss" appears in 3D. It is not a true form of agate, as it lacks agate's defining feature of concentric banding.

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Chrome chalcedony is a green variety of chalcedony, which is colored by chromium compounds. It is also known as "mtorolite" when found in Zimbabwe and "chiquitanita" when found in Bolivia.

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Onyx is a variant of agate with black and white banding. Similarly, agate with brown, orange, red and white banding is known as sardonyx.

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In Greenland, white to greyish chalcedony is known from volcanic strata of the Paleocene, in the Disko-Nuussuaq area (West Greenland) and from the Scoresby Sound area (East Greenland). A light blue variety of chalcedony is known from Illorsuit, formed in the volcanic rocks along the southern coast of the island. Because of its bluish, ice-like colour, it has the local name chalcedony "ice-blue".

[https://www.geus.dk/Media/637588304356429792/Smykkestensrapport_GEUS2021.pdf >Ghisler & Secher (2020) Smykkesten fra Grønland i GEUS' prøvesamlinger (Gemstones from Greenland in the sample collections of the Geological Survey of Denmark and Greenland), Danmarks og Grønlands Geologiske Untersøgelse Rapport 2020/50]

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File:Titus cameo.jpg of Titus head, 2nd century AD]]

Chalcedony was used in tool making as early as 32,000 BP in Central Australia where archaeological studies at sites in the Cleland Hills uncovered flakes from stone brought in from quarries many kilometres away.{{Cite journal |url= https://pdfs.semanticscholar.org/a0ef/cdca1322f84930b28bdb347b601ae04e27ff.pdf|archive-url= https://web.archive.org/web/20200722173108/https://pdfs.semanticscholar.org/a0ef/cdca1322f84930b28bdb347b601ae04e27ff.pdf|title=Characterizing Late Pleistocene and Holocene Stone Artefact Assemblages from Puritjarra Rock Shelter: A Long Sequence from the Australian Desert |last=Smith |first=M. |journal=Records of the Australian Museum |date=2006 |volume=58 |issue=3 |pages=371–410 |publisher=Australian Museum |doi=10.3853/J.0067-1975.58.2006.1470 |s2cid=86522736 |url-status=live |archive-date=2020-07-22 |hdl=1885/27417 |hdl-access=free }} Pre-contact uses described in the twentieth century included ceremonial stone knives.{{Cite book |title=The Archaeology of Australian Deserts |last=Smith |first=Mike |date=2013 |page=295}}

Chalcedony was used for green and yellow color in prehistoric cave paintings, for example at the Bhimbetka rock shelters. The chalcedony was ground to powder form then mixed with water and animal fat or tree resin or gum.{{Cite web |title=Rock Art of India – Pre and protohistoric cultures of India |url=https://ebooks.inflibnet.ac.in/icp02/chapter/rock-art-of-india/ |access-date=2024-05-03 |website=ebooks.inflibnet.ac.in}}

In the Bronze Age chalcedony was in use in the Mediterranean region; for example, on Minoan Crete at the Palace of Knossos, chalcedony seals have been recovered dating to circa 1800 BC.{{Cite web |url=http://www.themodernantiquarian.com/site/10854/knossos.html#fieldnotes |title=Knossos fieldnotes |last=Hogan |first=C. Michael |date=2007 |publisher=Modern Antiquarian |url-status=live |archive-url=https://web.archive.org/web/20160409225800/http://www.themodernantiquarian.com/site/10854/knossos.html#fieldnotes |archive-date=2016-04-09 }} People living along the Central Asian trade routes used various forms of chalcedony, including carnelian, to carve intaglios, ring bezels (the upper faceted portion of a gem projecting from the ring setting), and beads that show strong Greco-Roman influence.

Fine examples of first century objects made from chalcedony, possibly Kushan, were found in recent years at Tillya-tepe in north-western Afghanistan.Section 12 of the translation of Weilue – a 3rd-century Chinese text by John Hill under "carnelian" and note 12.12 (17)A. Also see Afghanistan's exhibition: [http://www.farlang.com/profiles/afghanistan-museum-kabul/product.2009-03-24.6644617360 Intaglio with depiction of a griffin, Chalcedony, 4th century BC, Afghanistan] {{webarchive |url=https://web.archive.org/web/20120225063934/http://www.farlang.com/profiles/afghanistan-museum-kabul/product.2009-03-24.6644617360 |date=February 25, 2012 }}

Hot wax would not stick to it so it was often used to make seal impressions.

The term chalcedony is derived from the name of the ancient Greek town Chalkedon in Asia Minor, in modern English usually spelled Chalcedon, today the Kadıköy district of Istanbul.

File:ChalcedonyKnifeNPS.jpg

According to tradition, at least three varieties of chalcedony were used in the Jewish High Priest's Breastplate. (Jewish tradition states that Moses' brother Aaron wore the Breastplate, with inscribed gems representing the twelve tribes of Israel.) The Breastplate supposedly included jasper, chrysoprase and sardonyx, and there is some debate as to whether other agates were also used.

In the 19th century, Idar-Oberstein, Germany, became the world's largest chalcedony processing center, working mostly on agates. Most of these agates were from Latin America, in particular Brazil. Originally the agate carving industry around Idar and Oberstein was driven by local deposits that were mined in the 15th century.{{Cite book |title=Precious Stones and Gems |last=Streeter |first=Edwin |date=1898 |page=237}} Several factors contributed to the re-emergence of Idar-Oberstein as agate center of the world: ships brought agate nodules back as ballast, thus providing extremely cheap transport. In addition, cheap labor and a superior knowledge of chemistry allowed them to dye the agates in any color with processes that were kept secret. Each mill in Idar-Oberstein had four or five grindstones. These were of red sandstone, obtained from Zweibrücken; and two men ordinarily worked together at the same stone.

File:Sumin chalcedony mouse.jpg]]

Chalcedony was once thought to be a fibrous variety of cryptocrystalline quartz.{{Cite web |url=http://www.mindat.org/min-960.html |title=Chalcedony mineral information and data |website=www.mindat.org |url-status=live |archive-url=https://web.archive.org/web/20060821000434/http://www.mindat.org/min-960.html |archive-date=2006-08-21 }} More recently however, it has been shown to also contain a monoclinic polymorph of quartz, known as moganite. The fraction, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20%.{{Cite journal |last1=Heaney |first1=Peter J. |last2=Post |first2=Jeffrey E. |date=24 January 1992 |title=The Widespread Distribution of a Novel Silica Polymorph in Microcrystalline Quartz Varieties |journal=Science |series=New Series |volume=255 |issue=5043 |pages=441–443 |doi=10.1126/science.255.5043.441 |pmid=17842895 |jstor=2876012|bibcode=1992Sci...255..441H |s2cid=32497622 }} The existence of moganite was once regarded as dubious, but it is now officially recognised by the International Mineralogical Association.{{Cite journal |last=Origlieri |first=Marcus |date=January 1994 |title=Moganite: a New Mineral – Not! |url=http://fgms.home.att.net/moganite.htm |journal=Lithosphere |archive-url=https://web.archive.org/web/20080309094336/http://fgms.home.att.net/moganite.htm |archive-date=March 9, 2008 }}{{Cite journal |last1=Nickel |first1=Ernest H. |last2=Nichols |first2=Monte C. |date=16 May 2008 |title=IMA/CNMNC List of Mineral Names |url=http://www.geo.vu.nl/users/ima-cnmmn/MINERALlist.pdf |url-status=dead |journal=Materials Data |archive-url=https://web.archive.org/web/20080530134400/http://www.geo.vu.nl/users/ima-cnmmn/MINERALlist.pdf |archive-date=2008-05-30 |access-date=2008-06-29}}>

Chalcedony is more soluble than quartz under low-temperature conditions, despite the two minerals being chemically identical. Possible reasons include the existence of the moganite component, defects caused by Brazil twinning, and small crystal size.{{cite journal |last1=Gislason |first1= S. R. |last2=Heaney |first2= P. J. |last3= Veblen |first3= D. R. |last4= Livi |first4= K. J. T. |journal=Chemical Geology |date=March 1993 |title=The difference between the solubility of quartz and chalcedony: the cause? |volume= 107 |issue= 3–4 |url= https://www.academia.edu/13667025 |pages=363–366 |doi=10.1016/0009-2541(93)90210-A |bibcode= 1993ChGeo.107..363G |access-date=2024-04-12}}

This table gives equilibrium concentrations of total dissolved silicon as calculated by PHREEQC (PH REdox EQuilibrium (in C language, USGS)) using the llnl.dat database.{{Citation needed|date=December 2009}}

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• List of minerals
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{{EB1911 poster|Chalcedony}}

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• [http://www.mindat.org/min-960.html Mindat: mineralogical data Chalcedony]
• [http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/chalcedony.html USGS: US Chalcedony locations] {{Webarchive|url=https://web.archive.org/web/20180701054945/https://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/chalcedony.html |date=2018-07-01 }}

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Category:Lithics

Category:Trigonal minerals