Chalcedony

{{Short description|Microcrystalline or cryptocrystalline form of silica}} {{use dmy dates|date=February 2026}} {{Infobox mineral | name = Chalcedony | category = Tectosilicate minerals | group = Quartz group | IMAstatus = Variety of quartz | image = Quartz-46809.jpg | imagesize = 260px | caption = A botryoidal geode specimen from Mexico | formula = {{chem|Si|O|2}} (silica) | molweight = 60 g/mol | color = Various | habit = Various | system = Trigonal (quartz) or 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 |year=1990 |publisher=Arch Cape Press}}</ref> }}

'''Chalcedony'''{{efn|Pronounced {{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 an aggregate of cryptocrystalline or microcrystalline silica, or in simpler terms, a mass of individual silica crystals that are too small to be seen without magnification. It is composed primarily of quartz, with minor amounts of intergrown 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 minerals with a chemical composition of {{chem2|SiO2}}, but they differ in that quartz has a more symmetrical crystal structure (trigonal) than moganite (monoclinic). Mineralogically speaking, the quartz and moganite crystals in chalcedony must grow in parallel chains, called "fibers." However, the term ''chalcedony'' may more broadly refer to any microcrystalline or cryptocrystalline silica, fibrous or not.

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://gemsociety.org/article/chalcedony-jewelry-and-gemstone-information |website=gemsociety.org |publisher=International Gem Society |access-date=2025-02-10}}</ref>

==Etymology== 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 |year=2007 |at=S. 307 |url={{Google books |plainurl=yes|id=OtaMtKzaKR8C |page=307}}}}</ref> According to the ''Online Etymology Dictionary'', however, a connection with the town of Chalcedon is "very doubtful".<ref>{{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=Naturalis Historiae |author=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=International Standard Bible Encyclopedia |editor=James Orr |publisher=Howard-Severance Co. |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=1927-11-01 |volume=12 |issue=11 |page=392 |url=http://minsocam.org/ammin/am12/am12_383.pdf |publisher=Mineralogical Society of America}}</ref>

Chalcedony is an aggregate of microscopic (microcrystalline) or submicroscopic (cryptocrystalline) silica crystals.<ref name="mindat">{{cite web |title=Chalcedony |url=https://www.mindat.org/min-960.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=26 April 2026}}</ref> The mineral species of silica present in chalcedony is typically quartz, although a percentage may be 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|url-access=subscription }}</ref><ref name="heany_1994" /> The existence of moganite was once regarded as dubious,<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> but it is now officially recognized by the International Mineralogical Association.<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>

All forms of silica, including quartz and moganite, have a chemical composition of {{chem2|SiO2}}. They differ in that quartz has greater symmetry, crystallizing in the trigonal crystal system while moganite crystallizes in the monoclinic crystal system.<ref name="mindat" /><ref>{{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 geological time, the metastable moganite is recrystallized as quartz.<ref name="Minerals 2020" /><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|url-access=subscription }}</ref>

Unlike macroscopic (macrocrystalline) quartz, which is anhydrous, chalcedony normally contains very small amounts of water.<ref name="chalcedony" />

==Structure== In mineralogical literature, chalcedony refers strictly to fibrous microcrystalline or cryptocrystalline quartz.<ref name="mindat" /><ref name="Minerals 2020" /> Chalcedony 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><ref name="Minerals 2020" /> giving them a helical shape.<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> Individual fibers in agates were determined by X-ray diffraction to be 0.1-1.0 µm in diameter and up to several millimeters long.<ref name="Minerals 2020">{{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 }}</ref>

There are two distinct types of chalcedony fibers. Length-fast chalcedony fibers consist of crystals stacked perpendicular to the c-axis (side to side).<ref name="chalcedony" /><ref name="Minerals 2020" /> These crystals are intergrown, polysynthetic twins according to Brazil law.<ref name="Minerals 2020" /> Length-slow chalcedony, also known as ''quartzine'', consists of quartz crystals stacked parallel to the c-axis (tip to tip).<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><ref name="Minerals 2020" />

==Properties== ===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 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="Heaney 1992" />

== Varieties == 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:

===Agate=== 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. {{clear}}

===Carnelian=== 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 sard, which is brown rather than red. {{clear}}

===Chert=== {{Main article|Chert}}

===Chrysoprase=== thumb|left|180px|Chrysoprase 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.)

=== Fire agate === 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}}

===Flint=== {{Main article|Flint}}

===Heliotrope=== thumb|left|180px|Heliotrope, or bloodstone 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. {{clear}}

===Jasper=== {{Main article|Jasper}}

===Moss agate=== 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. {{clear}}

===Chrome chalcedony=== 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. {{clear}}

===Onyx=== 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. {{clear}}

===Petrified wood=== {{Main article|Petrified wood}}

===Turritella agate=== {{Main article|Turritella agate}}

== History == [[File:Titus cameo.jpg|upright|thumb|Chalcedony cameo of Titus head, 2nd century AD]] Chalcedony was used in tool making as early as {{circa}} 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.<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 Crete at the Palace of Knossos, chalcedony seals have been recovered dating to circa 1800 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 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.<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 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.

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.

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

==Notes== {{notelist}}

==References== {{Reflist}}

==External links== {{EB1911 poster|Chalcedony}} {{commons category}} *[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 }}

Category:Chalcedony Category:Lithics Category:Trigonal minerals Category:Silica polymorphs Category:Monoclinic minerals