Chalcedony

Source: Wikipedia, the free encyclopedia.
Chalcedony
Specific gravity
2.59–2.61
References[1]

Chalcedony (

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 color of chalcedony sold commercially is often enhanced by dyeing or heating.[4]

The name chalcedony comes from the Latin

precious gem mentioned in Revelation is the same as the mineral known by this name today.[7] The term plasma is sometimes used to refer to green translucent chalcedony.[8]

A rare pseudomorph of a spiral Turritella-like snail shell that has been replaced by chalcedony

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

Agate

Agate is a variety of chalcedony characterized by either transparency or color patterns, such as multi-colored curved or angular banding. Opaque varieties are sometimes referred to as jasper.[4] 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.[9]

Carnelian

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.

Chrysoprase

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.)

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.

Fire agate

Fire agate

Fire agate is a variety of chalcedony with inclusions of goethite or limonite causing an iridescent effect.[10] It can display a wide range of iridescent colors including red, orange, yellow, green, blue, and purple.[11]

Heliotrope

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, the spots are yellow instead, known as plasma.

Moss agate

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.

Chrome chalcedony

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.

Onyx

Onyx

sardonyx
.

Chalcedony ice-blue

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". [12]

History

Chalcedony cameo 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.[13] Pre-contact uses described in the twentieth century included ceremonial stone knives.[14]

In the

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

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
.

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.[17] 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.[17]

Geochemistry

Chalcedony mouse, by Avenir Sumin

Structure

Chalcedony was once thought to be a fibrous variety of cryptocrystalline

monoclinic polymorph of quartz, known as moganite.[3] The fraction, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20%.[19] The existence of moganite was once regarded as dubious, but it is now officially recognised by the International Mineralogical Association.[20][21]

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.[22][19]

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]

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

References

  1. ^ Duda, Rudolf; Rejl, Lubos (1990). Minerals of the World. Arch Cape Press.
  2. ^ "chalcedony". Dictionary.com Unabridged (Online). n.d.
  3. ^ a b Heaney, Peter J. (1994). "Structure and Chemistry of the low-pressure silica polymorphs". In Heaney, P. J.; Prewitt, C. T.; Gibbs, G. V. (eds.). Silica: Physical Behavior, geochemistry and materials applications. Reviews in Mineralogy. Vol. 29. pp. 1–40.
  4. ^ a b "Chalcedony Gemological Information". International Gem Society (IGS). Archived from the original on 7 February 2015. Retrieved 16 January 2015.
  5. OED, however, a connection with the town of Chalcedon is "very doubtful":Harper, Douglas. "Chalcedony". Online Etymology Dictionary
    .
  6. ^ Pliny the Elder. "chapter 7". Naturalis Historiae. Vol. Book 37. p. 115.
  7. ^ James Orr, ed. (1915). "Chalkēdōn". The International Standard Bible Encyclopaedia. The Howard-Severance company. p. 2859.
  8. ^ Hart, Gilbert (1 Nov 1927). "The nomenclature of silica" (PDF). American Mineralogist. 12 (11). Mineralogical Society of America: 392.
  9. ^ CIBJO member laboratories (May 2009). "Retailers' Reference Guide: Diamonds, Cemstones, Pearls and Precious Metals". Bern, Switzerland: CIBJO (The World Jewellery Federation, international federation of all national trade organizations and gemological laboratories).
  10. ^ "Fire Agate". www.mindat.org. Retrieved 2021-04-11.
  11. ^ Page, Geology (2020-01-13). "Fire Agate : What is fire agate? How fire agate is formed?". Geology Page. Retrieved 2021-04-11.
  12. ^ >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
  13. S2CID 86522736. Archived
    (PDF) from the original on 2020-07-22.
  14. ^ Smith, Mike (2013). The Archaeology of Australian Deserts. p. 295.
  15. ^ Hogan, C. Michael (2007). "Knossos fieldnotes". Modern Antiquarian. Archived from the original on 2016-04-09.
  16. ^ 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 Archived February 25, 2012, at the Wayback Machine
  17. ^ a b Streeter, Edwin (1898). Precious Stones and Gems. p. 237.
  18. ^ "Chalcedony mineral information and data". www.mindat.org. Archived from the original on 2006-08-21.
  19. ^
    S2CID 32497622
    .
  20. ^ Origlieri, Marcus (January 1994). "Moganite: a New Mineral – Not!". Lithosphere. Archived from the original on March 9, 2008.
  21. ^ Nickel, Ernest H.; Nichols, Monte C. (16 May 2008). "IMA/CNMNC List of Mineral Names" (PDF). Materials Data. Archived from the original (PDF) on 2008-05-30. Retrieved 2008-06-29.>
  22. doi:10.1016/0009-2541(93)90210-A
    . Retrieved 2024-04-12.

External links

Wikisource has the text of the 1911 Encyclopædia Britannica article "Chalcedony".
Retrieved from "https://en.wikipedia.org/w/index.php?title=Chalcedony&oldid=1218627133"