Chalcedony (pronounced /kælˈsɛdəni/) is a cryptocrystalline Cryptocrystalline is a rock texture which is so finely crystalline, being made up of such minute crystals, that its crystalline nature is only vaguely revealed even microscopically in thin section by transmitted polarized light. Among the sedimentary rocks, chert and flint are cryptocrystalline. Carbonado, a form of diamond, is also form of silica The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of Si , composed of very fine intergrowths of the minerals quartz Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2 and moganite.^[2] These are both silica minerals A mineral is a naturally occurring solid chemical substance that is formed through geological processes and that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not have a specific chemical composition, but they differ in that quartz has a trigonal In crystallography, the trigonal crystal system is one of the seven crystal systems, and the rhombohedral lattice system is one of the seven lattice systems. They are often confused with each other: crystals in the rhombohedral lattice system are always in the trigonal crystal system, but some crystals such as quartz are in the trigonal crystal crystal structure, whilst moganite is monoclinic In crystallography, the monoclinic crystal system is one of the 7 lattice point groups. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal length, as in the orthorhombic system. They form a rectangular prism with a parallelogram as its base. Hence two pairs of vectors are.

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 mineral gets its name from the town of Chalcedon Chalcedon was an ancient maritime town of Bithynia, in Asia Minor, almost directly opposite Byzantium, south of Scutari (modern Üsküdar). It is now a district of the city of Istanbul named Kadıköy. The variant Calchedon (Greek: Καλχήδων; "copper") is found on all the coins of Chalcedon as well as in manuscripts of Herodotus's.

Varieties

Chalcedony occurs in a wide range of varieties. Many semi-precious gemstones A gemstone or gem is a piece of attractive mineral, which—when cut and polished—is used to make jewelry or other adornments. However certain rocks, (such as lapis lazuli) and organic materials (such as amber or jet) are not minerals, but are still used for jewelry, and are therefore often considered to be gemstones as well. Most gemstones are are in fact forms of chalcedony. The more notable varieties of chalcedony are as follows:

Agate

Agate Agate is a microcrystalline variety of quartz (silica), chiefly chalcedony, characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks but can be common in certain metamorphic rocks 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.^[3]

Aventurine

Aventurine is a form of quartz Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2, characterised by its translucency and the presence of platy mineral A mineral is a naturally occurring solid chemical substance that is formed through geological processes and that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not have a specific chemical composition inclusions Inclusions are usually other minerals or rocks, but may also be water, gas or petroleum. Liquid or vapor inclusions are known as fluid inclusions. In the case of amber it is possible to find insects and plants as inclusions that give a shimmering or glistening effect termed aventurescence In gemology, aventurescence is an optical reflectance effect seen in certain gems. The effect amounts to a metallic glitter, arising from minute, preferentially oriented mineral platelets within the material. These platelets are so numerous that they also influence the material's body colour. In aventurine quartz (actually a type of quartzite).

The most common colour of aventurine is green, but it may also be orange, brown, yellow, blue, or gray. Chrome-bearing fuchsite Muscovite is a phyllosilicate mineral of aluminium and potassium with formula K (a variety of muscovite Muscovite is a phyllosilicate mineral of aluminium and potassium with formula K mica The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic with a tendency towards pseudo-hexagonal crystals and are similar in chemical composition. The highly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-) is the classic inclusion, and gives a silvery green or blue sheen. Oranges and browns are attributed to hematite Hematite, also spelled as hæmatite, is the mineral form of iron oxide (Fe2O3), one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum. Hematite and ilmenite form a complete solid solution at temperatures above 950°C or goethite Goethite (FeO), (pronounced: "Gertite" /ˈgɝtaɪt/) named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low-temperature environments. Goethite has been well known since prehistoric times for its use as a pigment. Evidence has been found of its use in paint pigment samples. Because aventurine is a rock In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids, its physical properties vary: its specific gravity Relative density, or specific gravity, is the ratio of the density of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water. The term "relative density" is often preferred in modern scientific usage may lie between 2.64-2.69 and its hardness The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science. The method of comparing hardness by seeing which is somewhat lower than single-crystal quartz Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2 at around 6.5.

Aventurine feldspar or sunstone Sunstone is a plagioclase feldspar, which when viewed from certain directions exhibits a brilliant spangled appearance; this has led to its use as a gemstone. It has been found in Southern Norway, and in some United States localities. It is the official gemstone of Oregon can be confused with orange and red aventurine quartzite, although the former is generally of a higher transparency. Aventurine is often banded and an overabundance of fuchsite may render it opaque, in which case it may be mistaken for malachite Malachite is a copper carbonate mineral, with the formula Cu2CO3 at first glance.

The name aventurine derives from the Italian Italian ( italiano , or lingua italiana) is a Romance language spoken as a native language by about 62 million people in Italy, San Marino and parts of Switzerland, Croatia, Slovenia and France. It is spoken as a first language by many Italian citizens and immigrants abroad, for a total of approximately 70 million native speakers. In addition, it "a ventura" meaning "by chance". This is an allusion to the lucky discovery of aventurine glass or goldstone Goldstone is a type of glittering glass made in the presence of a reducing flame. The finished product can take a smooth polish and be carved into beads, figurines, or other artifacts suitable for semiprecious stone, and in fact goldstone is often mistaken for or misrepresented as a natural material at some point in the 18th century. Although it was known first, goldstone is now a common imitation of aventurine and sunstone. Goldstone is distinguished visually from the latter two minerals by its coarse flecks of copper Copper is a chemical element with the symbol Cu (Latin: cuprum) and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable, and a freshly exposed surface has a pinkish or peachy color. It is used as a thermal conductor, an electrical conductor, a building material, and a, dispersed within the glass in an unnaturally uniform manner. It is usually a golden brown, but may also be found in blue or green.

The majority of green and blue-green aventurine originates in India India, officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with 1.18 billion people, and the most populous democracy in the world. Mainland India is bounded by the Indian Ocean on the south, the Arabian Sea on the west, and the Bay of Bengal on the (particularly in the vicinity of Mysore Mysore (pronounced maɪˈsɔər in English; Kannada: ಮೈಸೂರು Maisūru; is the second largest city in the state of Karnataka, India. It is the headquarters of the Mysore district and the Mysore division and lies about 146 km (91 mi) southwest of Bangalore, the capital of Karnataka. The name Mysore is an anglicised version of Mahishūru, and Madras Chennai , formerly known as Madras (Tamil: மெட்ராஸ் AKA மதறாஸ்), is the capital city of the Indian state of Tamil Nadu. Chennai is the fourth most populous metropolitan area and the fifth most populous city in India. Located on the Coromandel Coast of the Bay of Bengal, Chennai city had a population of 4.34 million in) where it is employed by prolific artisans. Creamy white, gray and orange material is found in Chile Chile (traditional English pronunciation /ˈtʃɪli/, also pronounced /ˈtʃiːleɪ/ ), officially the Republic of Chile (Spanish: República de Chile [reˈpuβlika ðe ˈtʃile] ( listen)), is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders, Spain Spain (pronounced /ˈspeɪn/ spayn; Spanish: España, pronounced [esˈpaɲa] ( listen)), officially the Kingdom of Spain (Spanish: Reino de España), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula.[note 6] Its mainland is bordered to the south and east by the Mediterranean Sea except for and Russia Russia (pronounced /ˈrʌʃə/ ; Russian: Россия, tr. Rossiya, pronounced [rɐˈsʲijə] ( listen)), also officially known as the Russian Federation (Russian: Российская Федерация, tr. Rossiyskaya Federatsiya, pronounced [rɐˈsʲijskəjə fʲɪdʲɪˈraʦəjə] ( listen)), is a country in northern Eurasia. It is a federal. Most material is carved into beads and figurines with only the finer examples fashioned into cabochons A cabochon or cabachon, from the Middle French caboche , is a gemstone which has been shaped and polished as opposed to faceted. The resulting form is usually a convex top with a flat bottom. Cutting en cabochon is usually applied to opaque gems, while facetting is usually applied to transparent stones. Hardness is also taken into account as, later being set into jewellery Jewellery or jewelry (see American and British English spelling differences) signifies items of personal adornment, such as necklaces, rings, brooches, earrings and bracelets. Jewellery may be made from any material, such as gemstones, precious metals or shells, besides other materials, depending on cultural differences and availability of.

Main markets for aventurine are landscape stone, building stone, aquaria, monuments, and jewellery.

Carnelian

Carnelian Carnelian is a reddish-brown mineral which is commonly used as a semi-precious gemstone. Similar to carnelian is sard, which is generally harder and darker. (The difference is not rigidly defined, and the two names are often used interchangeably.) Both carnelian and sard are varieties of the silica mineral chalcedony colored by impurities of iron (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.

Chrysoprase

Chrysoprase Chrysoprase or chrysophrase is a gemstone variety of chalcedony that contains small quantities of nickel. Its color is normally apple-green, but varies to deep green. The darker varieties of chrysoprase are also referred to as prase. (However, the term prase is also used to describe chlorite-included quartz, and to a certain extent is a color- (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 Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2, and to a certain extent is a color-descriptor, rather than a rigorously defined mineral variety.)

Heliotrope

Heliotrope The mineral heliotrope, also known as bloodstone, is a form of chalcedony . The "classic" bloodstone is green chalcedony with red inclusions of iron oxide or red jasper. Sometimes the inclusions are yellow, in which case the mineral is given the name plasma is a green variety of chalcedony, containing red inclusions of iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Altogether, there are sixteen known iron oxides and oxyhydroxides. These inclusions resemble drops of blood Blood is a specialized bodily fluid that delivers necessary substances to the body's cells – such as nutrients and oxygen – and transports waste products away from those same cells, giving heliotrope its alternative name of bloodstone. A similar variety, in which the spots are yellow instead of red is known as plasma.

Moss agate

Moss agate Moss agate is a semi-precious gemstone formed from silicon dioxide. This mineral is commonly found in limestone and granite. It is a form of agate which includes minerals of a green colour embedded in the chalcedony, forming filaments and other patterns suggestive of moss. It also sometimes resembles blue-cheese. The field is a clear or milky- (also known as tree agate or mocha stone) contains green filament-like inclusions, giving it the superficial appearance of moss Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems. At certain times mosses produce spore capsules which may appear as beak-like or blue cheese Blue cheese is a general classification of cow's milk, sheep's milk, or goat's milk cheeses that have had Penicillium cultures added so that the final product is spotted or veined throughout with blue, blue-gray or blue-green mold, and carries a distinct smell. Some blue cheeses are injected with spores before the curds form and others have spores. It is not a true form of agate, as it lacks agate's defining feature of concentric banding.

Mtorolite

Mtorolite is a green variety of chalcedony, which has been colored by chromium Chromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable. The name of the element is derived from the Greek word "chrōma" (χρώμα), meaning color,. It is principally found in Zimbabwe Zimbabwe is a landlocked country located in the southern part of the continent of Africa, between the Zambezi and Limpopo rivers. It is bordered by South Africa to the south, Botswana to the southwest, Zambia to the northwest and Mozambique to the east. Zimbabwe has three official languages: English, Shona (a Bantu language), and Ndebele (also a.

Onyx

Onyx Onyx is a cryptocrystalline form of quartz. The colors of its bands range from white to almost every color . Commonly, specimens of onyx available contain bands of colors of white, tan, and brown is a variant of agate with black and white banding. Similarly, agate with brown and white banding is known as sardonyx Onyx is a cryptocrystalline form of quartz. The colors of its bands range from white to almost every color . Commonly, specimens of onyx available contain bands of colors of white, tan, and brown. Sardonyx is a variant in which the colored bands are sard (shades of red) rather than black. Pure black onyx is common, and perhaps the most famous.

History

As early as 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.^[4] 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 Graeco-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.^[5] 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.

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 became the world's largest chalcedony processing center, in particular agates. Most of these agates were sourced in 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.^[6] 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. Cheap labor and a superior knowledge of chemistry allowing 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.^[6]

Geochemistry

Structure

Chalcedony was once thought to be a fibrous variety of cryptocrystalline quartz.^[7] More recently however, it has been shown to also contain a monoclinic polymorph of quartz, known as moganite.^[2] The fraction, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20%.^[8] The existence of moganite was once regarded as dubious, but it is now officially recognised by the International Mineralogical Association.^[9][10]

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]} It has also been suggested that the higher solubility is due to the moganite component.^[8]

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]}.

See also

• List of minerals

References

1. ^ Rudolf Duda and Lubos Rejl: Minerals of the World (Arch Cape Press, 1990)
2. ^ ^{a b} 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
3. ^ 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 CIBJO member laboratories
4. ^ C. Michael Hogan, Knossos fieldnotes, Modern Antiquarian (2007)
5. ^ 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:Intaglio with depiction of a griffin, Chalcedony, 4th century BC, Afghanistan
6. ^ ^{a b} Streeter, Edwin, 1898. Precious Stones and Gems. Page 237
7. ^ Chalcedony mineral information and data. http://www.mindat.org/min-960.html
8. ^ ^{a b} 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
9. ^ Origlieri, Marcus. "Moganite: a New Mineral -- Not!" Lithosphere (1994). Aug. 2007 <http://fgms.home.att.net/moganite.htm>.
10. ^ 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>

External links

• Mindat: mineralogical data Chalcedony
• USGS: US Chalcedony locations

Categories: Minerals | Quartz varieties | Lithics

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