Structure : Triclinic
Hardness : 6
Specific Gravity : 2.8
Refractive index : 1.61 - 1.6
Doubly Refracting (bi-refraction) : 0.04

Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium, with the chemical formula CuAl6(PO4)4(OH)8 4H2O. It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue. In recent times turquoise, like most other opaque gems, has been devalued by the introduction of treatments, imitations, and synthetics onto the market.

The substance has been known by many names, but the word turquoise was derived around the 16th century from the French language either from the word for Turkish (Turquois) or dark-blue stone (pierre turquin). This may have arisen from a misconception: turquoise does not occur in Turkey but was traded at Turkish bazaars to Venetian merchants who brought it to Europe. In addition, the colour has been employed extensively in the decorative tiles adorning Turkish places of worship and homes at least since the 14th century. Another conjecture is that the name refers to the colour of the Mediterranean Sea on the southern Turkish coast.