View source for Chalcedony

{{Short description|Microcrystalline varieties of silica}}
{{Infobox mineral
| name          = Chalcedony
| category      = [[Tectosilicate]] [[minerals]], [[quartz]] [[Mineral group|group]]
| image         = Quartz-46809.jpg
| imagesize     = 260px
| caption       = 
| formula       = [[Silica]] (silicon dioxide, {{Chem|Si|O|2}})
| molweight     = 60 g/mol
| color         = Various
| habit         =
| system        = [[Hexagonal crystal family|Trigonal]] ([[quartz]]) or [[Monoclinic crystal system|monoclinic]] ([[moganite]])
| twinning      =
| cleavage      = Absent
| fracture      = Uneven, splintery, conchoidal
| mohs          = 6–7
| luster        = Waxy, vitreous, dull, greasy, silky
| refractive    =
| opticalprop   =
| birefringence =
| pleochroism   =
| streak        = White
| gravity       = 2.59–2.61
| density       =
| melt          =
| fusibility    =
| diagnostic    =
| solubility    =
| diaphaneity   = Translucent
| other         =
| references    = <ref>{{Cite book |title=Minerals of the World |last1=Duda |first1=Rudolf |last2=Rejl |first2=Lubos |date=1990 |publisher=Arch Cape Press}}</ref>
}}

'''Chalcedony''' ({{IPAc-en|k|æ|l|ˈ|s|ɛ|d|ə|n|i}} {{respell|kal|SED|ə|nee}} or {{IPAc-en|ˈ|k|æ|l|s|ə|ˌ|d|oʊ|n|i}} {{respell|KAL|sə|doh|nee}})<ref>{{cite Dictionary.com|chalcedony}}</ref> is a [[cryptocrystalline]] form of [[silica]], composed of very fine intergrowths of [[quartz]] and [[moganite]].<ref name="heany_1994">{{Cite book |title=Silica: Physical Behavior, geochemistry and materials applications |last=Heaney |first=Peter J. |date=1994 |editor-last=Heaney |editor-first=P. J. |series=Reviews in Mineralogy |volume=29 |pages=1–40 |chapter=Structure and Chemistry of the low-pressure silica polymorphs |editor-last2=Prewitt |editor-first2=C. T. |editor-last3=Gibbs |editor-first3=G. V.}}</ref> These are both silica [[mineral]]s, but they differ in that quartz has a [[trigonal]] crystal structure, while moganite is [[monoclinic]]. Chalcedony's standard [[chemical structure]] (based on the chemical composition of quartz) is SiO<sub>2</sub> ([[silicon dioxide]]).

Chalcedony has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colors, but those most commonly seen are white to gray, grayish-blue or a shade of brown ranging from pale to nearly black. The color of chalcedony sold commercially is often enhanced by dyeing or heating.<ref>{{cite web |title=Chalcedony Value, Price, and Jewelry Information |url=https://www.gemsociety.org/article/chalcedony-jewelry-and-gemstone-information/ |website=gemsociety.org |publisher=International Gem Society |access-date=10 February 2025}}</ref>

The name ''chalcedony'' comes from the Latin {{wikt-lang|la|chalcedonius}} (alternatively spelled {{lang|la|calchedonius}}) and is probably derived from the town of [[Chalcedon]] in [[Asia Minor]].<ref>{{cite book| first=Erika |last=Zwierlein-Diehl|title=Antike Gemmen und ihr Nachleben|location=Berlin |publisher =Verlag [[Walter de Gruyter]]|date=2007|at=S. 307 |url={{google books|plainurl=yes|id=OtaMtKzaKR8C|page=307}}}} According to the [[OED]], however, a connection with the town of Chalcedon is "very doubtful":{{OEtymD|Chalcedony}}</ref> The name appears in [[Pliny the Elder]]'s {{lang|la|[[Naturalis Historia]]|italics=set}} as a term for a translucent kind of [[jaspis]].<ref>{{Cite book |chapter-url={{google books|id=oHlGAAAAYAAJ|page=163|plainurl=yes}} |title=[[Natural History (Pliny)|Naturalis Historiae]] |last=Pliny the Elder |volume=Book 37 |page=115 |chapter=chapter 7}}</ref>  Another reference to a gem by the name of {{Transliteration|grc|khalkedón}} ({{wikt-lang|grc|χαλκηδών}}) is found in the [[Book of Revelation]] (21:19); however, it is a ''[[hapax legomenon]]'', found nowhere else in the Bible, so it is hard to tell whether the [[precious gem]] mentioned in Revelation is the same as the mineral known by this name today.<ref>{{cite book |title=The International Standard Bible Encyclopaedia |editor=James Orr |publisher=The Howard-Severance company |year=1915 |chapter=''Chalkēdōn'' |page=2859 |chapter-url=https://books.google.com/books?id=CqPNAAAAMAAJ&pg=PA2859}}</ref> The term ''plasma'' is sometimes used to refer to green translucent chalcedony.<ref>{{cite journal |last1=Hart |first1=Gilbert |title=The nomenclature of silica |journal=American Mineralogist |date=1 Nov 1927 |volume=12 |issue=11 |page=392 |url=http://www.minsocam.org/ammin/am12/am12_383.pdf |publisher=Mineralogical Society of America}}</ref>

== Varieties ==

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

===Agate===
[[File:Quartz - Agateplate, redbrown-white.jpg|thumb|left|180px|Agate]]
[[Agate]] is a fibrous, banded variety of chalcedony that occurs in a variety of colors and patterns.<ref>{{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}}</ref> 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.<ref name="cibjo">{{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 }}</ref>
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===Carnelian===
[[File:carnelian sard (mineral specimen).jpg|thumb|left|180px|Carnelian]]
[[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 [[carnelian|sard]], which is brown rather than red.
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===Chrysoprase===
[[File:Chryzopras Polsko.jpg|thumb|left|180px|Chrysoprase]]
[[Chrysoprase]] (also spelled chrysophrase) is a green variety of chalcedony, which has been colored by [[Nickel(II) oxide|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 ===
[[File:Agate-180670.jpg|left|thumb|Fire agate]]

[[Fire agate]] is a variety of chalcedony with inclusions of goethite or limonite causing an iridescent effect.<ref>{{Cite web|title=Fire Agate|url=https://www.mindat.org/min-7601.html|access-date=2021-04-11|website=www.mindat.org}}</ref> It can display a wide range of iridescent colors including red, orange, yellow, green, blue, and purple.<ref>{{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}}</ref> {{Clear}}

===Heliotrope===
[[File:Quarz - Heliotrop (Blutjaspis).JPG|thumb|left|180px|Heliotrope, or bloodstone]]
[[Heliotrope (mineral)|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===
[[File:mossagate.pebble.750pix.jpg|thumb|left|180px|Tree agate]]
[[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===
[[File:Mtorolite zimbabwe trimmed.jpg|thumb|left|Mtorolite]]
[[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===
[[File:Agate-Quartz-49959.jpg|thumb|left|180px|Onyx]]
[[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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===Chalcedony ice-blue===
[[File:Iceblue-all2_hg.jpg|thumb|left|180px|Chalcedony ''Ice-blue'' (boulder, polished section and cabochon in ring]]
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".
<ref>{{Cite web |url=https://www.geus.dk/Media/637588304356429792/Smykkestensrapport_GEUS2021.pdf |title=>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 |access-date=2022-06-19 |archive-date=2022-08-14 |archive-url=https://web.archive.org/web/20220814130930/https://www.geus.dk/Media/637588304356429792/Smykkestensrapport_GEUS2021.pdf |url-status=dead }}</ref>
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== History ==
[[File:Titus cameo.jpg|upright|thumb|Chalcedony [[Cameo (carving)|cameo]] of [[Titus]] head, 2nd century AD]]
Chalcedony was used in tool making as early as {{circa}} 32,000 [[Before Present|BP]] in [[Central Australia]] where archaeological studies at sites in the Cleland Hills uncovered flakes from stone brought in from quarries many kilometres away.<ref>{{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 }}</ref> Pre-contact uses described in the twentieth century included ceremonial stone knives.<ref>{{Cite book |title=The Archaeology of Australian Deserts |last=Smith |first=Mike |date=2013 |page=295}}</ref>

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.<ref>{{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}}</ref>

In the [[Bronze Age]] chalcedony was in use in the [[Mediterranean]] region; for example, on [[Minoan civilization|Minoan]] [[Crete]] at the Palace of [[Knossos]], chalcedony [[Minoan seal-stones|seal]]s have been recovered dating to circa 1800 [[Before Christ|BC]].<ref>{{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 }}</ref> People living along the [[Central Asia]]n trade routes used various forms of chalcedony, including [[carnelian]], to carve [[Intaglio (jewellery)|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 Empire|Kushan]], were found in recent years at [[Tillya-tepe]] in north-western [[Afghanistan]].<ref>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 }}
</ref> Hot wax would not stick to it so it was often used to make [[Seal (emblem)|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|left|thumb|upright=1.3|Chalcedony knife, AD 1000–1200]]
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.<ref name="streeter1">{{Cite book |title=Precious Stones and Gems |last=Streeter |first=Edwin |date=1898 |page=237}}</ref> 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.<ref name="streeter1" />

== Geochemistry==
[[File:Mogánite-798422.jpg|upright=0.5|thumb|A rare [[pseudomorph]] of a spiral ''[[Turritella]]''-like snail shell that has been replaced by chalcedony]]
[[File:Sumin chalcedony mouse.jpg|thumb|Chalcedony mouse, by [[Avenir Sumin]]]]

===Structure===
Chalcedony was once thought to be a fibrous variety of cryptocrystalline [[quartz]].<ref>{{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 }}</ref>  More recently however, it has been shown to also contain a [[monoclinic]] polymorph of quartz, known as [[moganite]].<ref name="heany_1994" />  The fraction, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20%.<ref name="heany_1992">{{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 }}</ref>  The existence of moganite was once regarded as dubious, but it is now officially recognised by the [[International Mineralogical Association]].<ref>{{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 }}</ref><ref>{{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}}></ref>

===Solubility===
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 [[Crystal twinning|Brazil twinning]], and small crystal size.<ref name="Gislason_1993">{{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}}</ref><ref name="heany_1992" />

====Solubility of quartz and chalcedony in pure water====
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}}
{{table alignment}}
{| class="wikitable col2right col3right"
|+Comparative solubility (mg/L)
! Temperature
! Quartz
! Chalcedony
|-
!  0.01&nbsp;°C
| 0.68
| 1.34
|-
!  25.0&nbsp;°C
| 2.64
| 4.92
|-
!  50.0&nbsp;°C
| 6.95
| 12.35
|-
!  75.0&nbsp;°C
| 14.21
| 24.23
|-
!  100.0&nbsp;°C
| 24.59
| 40.44
|}

== See also ==
{{Portal|Minerals}}
* [[List of minerals]]
* [[Moganite]]

==References==
{{Reflist}}

==External links==
{{EB1911 poster|Chalcedony}}
*{{commons category-inline}}
*[http://www.mindat.org/min-960.html Mindat: mineralogical data Chalcedony]
*[https://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 }}

{{Silica minerals}}
{{Authority control}}

[[Category:Chalcedony| ]]
[[Category:Lithics]]
[[Category:Trigonal minerals]]