Chalcedony
- "Bloodstone" redirects here.
Chalcedony is one of the cryptocrystalline varieties of the mineral quartz, having a waxy luster. Chalcedony may be semitransparent or translucent and is usually white to gray, grayish-blue or some shade of brown, sometimes nearly black. Other shades have been given different names. A clear red chalcedony is known as carnelian or sard; a green variety colored by nickel oxide is called chrysoprase. Prase is a dull green and onyx is black and white banded. Plasma is a bright to emerald-green chalcedony that is sometimes found with small spots of jasper resembling blood drops; it has been referred to as blood stone or heliotrope. Flint is also a variety of chalcedony.
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. 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.
Geochemistry: Chalcedony
Chalcedony is more soluble than quartz under low-temperature conditions. It is, however, crystallographically identical to quartz. A mineral with the exact same crystallographic and chemical properties should have the same bulk thermodynamic properties (properties which are used to predict solubility). But chalcedony is extremely finely grained (cryptocrystalline), and has a very high surface area to volume ratio. It is believed that this property of chalcedony is responsible for its higher solubility. [citation needed]
Onyx
Onyx is a banded variety of chalcedony, a cryptocrystalline form of quartz. The colors of its bands range from white to almost every color (not including shades such as purple, blue, or black). Commonly, specimens of onyx available contain colors of white, tan, and brown. Sardonyx is a variant in which the colored bands are sard (shades of red) rather than black.
It is usually cut as a cabochon, or into beads, and is also used for intaglios and cameos, where the bands make the image contrast with the ground. Some onyx is natural but much is produced by the staining of agate.
One can view a close variant of this material in St.Mary's Church at Żebbuġ, Gozo.
The name has sometimes been used, incorrectly, to label other banded lapidary materials, such as banded calcite found in Mexico, Pakistan, and other places, and often carved, polished and sold. This material is much softer than true onyx, and much more readily available. The majority of carved items sold as 'Onyx' today are this carbonate material. [1]
Chemical composition and name | SiO2 - Silicon dioxide |
Hardness (Mohs scale) | 7 |
Specific gravity | 2.65 - 2.667 |
Refractive index (R.I.) | 1.543 - 1.552 to 1.545 - 1.554 |
Birefringence | 0.009 |
Optic sign | Positive |
Optical character | Uniaxial |
Culture and historical/mythical usage
According to Rebbenu Bachya, the word Shoham in the verse Exodus 28:20 means "Onyx" and was the stone on the Ephod representing the Tribe of Joseph. There were also two Shoham stones on the shoulders of the Ephod.
In the Dreamlands, a fictional location in the works of the author H. P. Lovecraft, the palaces of Kadath are built of onyx.
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.
Temperature | Quartz Solubility (mg/L) | Chalcedony Solubility (mg/L) |
---|---|---|
0.01ºC | 0.68 | 1.34 |
25.0ºC | 2.64 | 4.92 |
50.0ºC | 6.95 | 12.35 |
75.0ºC | 14.21 | 24.23 |
100.0ºC | 24.59 | 40.44 |
See also
- List of minerals
ReferencesISBN links support NWE through referral fees
External links
- http://www.mindat.org/min-960.html
- http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/chalcedony.html
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