Turquoise

Turquoise (or turquois) is opaque, blue-to-green hydrated copper aluminium phosphate mineral according to the chemical formula CuAl₆(PO₄)₄(OH)₈·5H₂O. more...

It is rare and valuable in finer grades and has been enjoyed 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 had its popularity undermined by the introduction of treatments, imitations, and synthetics onto the market, some difficult to detect even by experts.

The word turquoise is very old and likely is derived from the French pierre turquoise, meaning Turkish stone. This is thought to have arisen from a misconception: turquoise does not occur in Turkey but was traded there, and the gem became associated with the country in the West.

Properties

Even the finest of turquoise is fracturable, reaching a maximum hardness of just under 6, or slightly less than window glass. Characteristically a cryptocrystalline mineral, turquoise almost never forms single crystals and all of its properties are highly variable. Its crystal system is proven to be triclinic via X-ray diffraction testing. With lower hardness comes lower specific gravity (high 2.90, low 2.60) and greater porosity: These properties are dependent on grain size. The lustre of turquoise is typically waxy to subvitreous, and transparency is usually opaque, but may be semitranslucent in thin sections. Colour is as variable as the mineral's other properties, ranging from white to a powder blue to a sky blue, and from a blue-green to a yellowish green. The blue is attributed to idiochromatic copper while the green may be the result of either iron impurities (replacing aluminium) or dehydration.

The refractive index (as measured by sodium light, 589.3 nm) of turquoise is approximately 1.61 or 1.62; this is a mean value seen as a single reading on a gemmological refractometer, owing to the almost invariably polycrystalline nature of turquoise. A reading of 1.61–1.65 (birefringence 0.040, biaxial positive) has been taken from rare single crystals. An absorption spectrum may also be obtained with a hand-held spectroscope, revealing a line at 432 nanometres and a weak band at 460 nanometres (this is best seen with strong reflected light). Under longwave ultraviolet light, turquoise may occasionally fluoresce green, yellow or bright blue; it is inert under shortwave ultraviolet and X-rays.

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