Turquoise

Turquoise is a phosphate mineral composed of copper, aluminium, phosphorus, and water. Its standard chemical formula is written CuAl6(PO4)4(OH)8·4H2O. Its characteristic colour ranges from blue to blue-green, and it is among the earliest gemstones known to have been used for ornament throughout human history.

Turquoise is not a hard mineral: its hardness on the Mohs scale is around 5 to 6, making it relatively easy to scratch. It is also porous to water, oils, and chemicals, so its colour can change when exposed to these over prolonged periods. For this reason, turquoise is typically stabilized before sale.

Formation and occurrence

Turquoise forms in the oxidized zones near the Earth's surface, where copper-bearing water percolating downward encounters rocks containing aluminium and phosphorus. It is therefore commonly found in the vicinity of copper mines, occurring in thin seams or nodules within host rock. Major producing countries include Iran, China, parts of Italy, and the southwestern United States.

Name and history

The English and French name 'turquoise' derives from a French word meaning 'Turkish', because the stone was historically imported from Iran through Turkish markets. In Arabic it is called 'fayruz'. Turquoise appears in ornaments from ancient Egypt and Mesopotamian cultures, as well as in indigenous cultures of North America.

Use and market

Turquoise is mainly used in jewellery: bracelets, rings, and ornamental dress and weapons. Its value depends on colour (pure bright blue without too much green is generally more valued), colour stability, and the presence of dark or brown veining or matrix. The market includes substitutes and imitations made from other materials such as dyed howlite.