Chalcedony

Chalcedony knife, AD 1000-1200

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 yellow spots of jasper resembling, and Bloodstone is similar to Plasma but with red spots of jasper, thus the name Bloodstone, it is also called heliotrope. Flint is also a variety of chalcedony.

Chalcedony cameo of Titus head, 2nd Century AD

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.

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

References

• See Section 12 of the translation of Weilue - a 3rd century Chinese text by John Hill under "carnelian" and note 12.12 (17)

See also

• List of minerals

External links

• http://www.mindat.org/min-960.html
• http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/chalcedony.html