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'{{Infobox mineral | name = Chalcedony | category = Oxide mineral | image = Chalcedony geode.JPG | imagesize = 250 | caption = A cut and polished Chalcedony [[geode]] | formula = [[Silica]] (silicon dioxide, SiO₂) | molweight = 60 g / mol | color = Various | habit = | system = Trigonal, Monoclinic | 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>Rudolf Duda and Lubos Rejl: ''Minerals of the World'' (Arch Cape Press, 1990)</ref> }} '''Chalcedony''' {{IPAc-en|k|æ|l|ˈ|s|ɛ|d|ə|n|i}} is a [[cryptocrystalline]] form of [[silica]], composed of very fine intergrowths of the minerals [[quartz]] and [[moganite]].<ref name="heany_1994">Heaney, Peter J., 1994. Structure and Chemistry of the low-pressure silica polymorphs. In: Reviews in Mineralogy v. 29; Silica: Physical Behavior, geochemistry and materials applications. Ed. Heaney, P.J., Prewitt, C.T., Gibbs, G.V., 1-40</ref> These are both silica [[minerals]], but they differ in that quartz has a [[trigonal]] crystal structure, while moganite is [[monoclinic]]. Chalcedony's standard [[chemical structure]] (based on the chemical structure of quartz) is SiO₂ (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 name ''chalcedony'' comes from the [[Latin]] ''chalcedonius'' (alternatively spelled ''calchedonius''). The name appears first in [[Pliny the Elder]]'s ''[[Naturalis Historia]]'' as a term for a translucid kind of Jaspis.<ref>Pliny the Elder: Naturalis Historiae, Book 37, chapter 7,115 ([http://books.google.de/books?id=oHlGAAAAYAAJ&dq=editions%3A2wxns2VZC7sC&hl=de&pg=RA1-PA163#v=onepage&q&f=false online])</ref> The name is probably derived from the town [[Chalcedon]] in [[Asia Minor]].<ref>Erika Zwierlein-Diehl: Antike Gemmen und ihr Nachleben. Berlin (Verlag [[Walter de Gruyter]]) 2007, S. 307 ([http://books.google.de/books?id=OtaMtKzaKR8C&pg=PA307&lpg=PA307&dq=chalcedon+Plinius&source=bl&ots=xnCuPP5Hl8&sig=Ay5ipzitxzqb3KQLnUTICrmk1uo&hl=de&sa=X&ei=NsnkUMg-yNe0BueHgJgM&ved=0CDQQ6AEwAQ#v=onepage&q=chalcedon%20Plinius&f=false online]) According to the [[OED]] a connection with the town of Chalcedon is "very doubtful":{{OEtymD|Chalcedony}}</ref> A little later the Greek word ''khalkedon'' (χαλκηδών) appears in the [[Book of Revelation]] (Apc 21,19). It is a [[Hapax legomenon]], a word found nowhere else, so it is impossible to tell whether the precious gem mentioned in the Bible is the same 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 |url=http://books.google.com/?id=CqPNAAAAMAAJ&pg=PA2859&dq=chalcedony+Chalkedon#v=onepage&q=chalcedony%20Chalkedon&f=false}}</ref> {{TOC limit|limit=3}} == 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 variety of chalcedony with multi-colored curved or angular banding. Fire agate shows iridescent phenomena on a brown background; 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">CIBJO (The World Jewellery Federation, international federation of all national trade organizations and gemological laboratories), Retailers' Reference Cuide: Diamonds, Cemstones, Pearls and Precious Metals, May 2009, Bern, Switzerland [http://www.cibjo.org/index.php?option=com_content&view=article&id=55&Itemid=32 CIBJO member laboratories]</ref> <br clear="all" /> ===Aventurine=== [[File:Aventurine.jpg|thumb|left|180px|Aventurine (''Unknown scale'')]] '''[[Aventurine]]''' is a form of quartz, characterised by its translucency and the presence of platy [[mineral]] [[inclusion (mineral)|inclusion]]s that give a shimmering or glistening effect termed [[aventurescence]]. Chrome-bearing [[fuchsite]] (a variety of [[muscovite]] [[mica]]) is the classic inclusion, and gives a silvery green or blue sheen. Oranges and browns are attributed to [[hematite]] or [[goethite]]. <br clear="all" /> ===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. <br clear="all" /> ===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.) <br clear="all" /> ===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]]. These inclusions resemble drops of [[blood]], giving heliotrope its alternative name of bloodstone. A similar variety, in which the spots are yellow instead of red is known as plasma. <br clear="all" /> ===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. <br clear="all" /> ===Mtorolite=== [[File:Mtorolite zimbabwe trimmed.jpg|thumb|left|180px|Mtorolite]] '''[[Mtorolite]]''' is a green variety of chalcedony, which has been colored by [[chromium]]. Also known as [[chrome chalcedony]], it is principally found in [[Zimbabwe]]. <br clear="all" /> ===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]]. <br clear="all" /> == History == [[File:Titus cameo.jpg|right|thumb|Chalcedony [[Cameo (carving)|cameo]] of [[Titus]] head, 2nd Century AD]] As early as 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>C. Michael Hogan, [http://www.themodernantiquarian.com/site/10854/knossos.html#fieldnotes ''Knossos fieldnotes'', Modern Antiquarian (2007)]</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]], 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] </ref> Hot wax would not stick to it so it was often used to make [[Seal (device)|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|300px|Chalcedony knife, AD 1000-1200]] At least three varieties of chalcedony were used in the Jewish High Priest's Breastplate. (Moses' brother Aaron wore the Breastplate, with inscribed gems representing the twelve tribes of Israel). The Breastplate 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, in particular 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">Streeter, Edwin, 1898. Precious Stones and Gems. 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. Also, cheap labor and a superior knowledge of chemistry allowing them to dye the agates in any color with processes that were kept secret helped. 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== ===Structure=== Chalcedony was once thought to be a fibrous variety of cryptocrystalline [[quartz]].<ref>Chalcedony mineral information and data. http://www.mindat.org/min-960.html</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">Heaney, Peter J., and Jeffrey E. Post. "The Widespread Distribution of a Novel Silica Polymorph in Microcrystalline Quartz Varieties." Science ns 255 (1992): 441-443. JSTOR. Aug. 2007. Keyword: moganite</ref> The existence of moganite was once regarded as dubious, but it is now officially recognised by the [[International Mineralogical Association]].<ref>Origlieri, Marcus. "Moganite: a New Mineral -- Not!" Lithosphere (1994). Aug. 2007 <http://fgms.home.att.net/moganite.htm>.</ref><ref>Nickel, Ernest H., and Monte C. Nichols. "IMA/CNMNC List of Mineral Names." Materials Data. June 2007. Aug. 2007 <http://www.geo.vu.nl/users/ima-cnmmn/MINERALlist.pdf></ref> ===Solubility=== Chalcedony is more [[soluble]] than quartz under low-temperature conditions, despite the two minerals being chemically identical. This is thought to be because chalcedony is extremely finely grained (cryptocrystalline), and so has a very high surface area to volume ratio. {{Citation needed|date=February 2007}} It has also been suggested that the higher solubility is due to the moganite component.<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]] using the llnl.dat database{{Citation needed|date=December 2009}}. {| class="wikitable" style="margin:auto" |- ! Temperature ! Quartz Solubility (mg/L) ! Chalcedony Solubility (mg/L) |- ! 0.01°C | 0.68 | 1.34 |- ! 25.0°C | 2.64 | 4.92 |- ! 50.0°C | 6.95 | 12.35 |- ! 75.0°C | 14.21 | 24.23 |- ! 100.0°C | 24.59 | 40.44 |} == See also == * [[List of minerals]] ==References== <references /> ==External links== {{Wikisource1911Enc|Chalcedony}} *{{commonscat-inline}} *[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] {{Silica minerals}} [[Category:Quartz varieties]] [[Category:Chert]] [[Category:Lithics]] [[Category:Trigonal minerals]]'