Turquoise is a hydrated copper aluminium phosphate. The copper supplies the blue, and iron substituting for aluminium pulls it toward green, which is why turquoise ranges continuously from sky blue to yellowish green and why the greener material tends to come from iron-rich deposits.
It is a comparatively soft and porous stone, around 5 to 6 on the Mohs scale, and this is the central practical fact about it. Turquoise absorbs oils, cosmetics and skin secretions, and it changes colour with wear: drifting green, sometimes over years, sometimes rapidly. The old belief that turquoise warns its owner of illness or danger by changing colour has an obvious physical basis: the stone genuinely does change, in response to the wearer's body chemistry and to what it has been exposed to. A belief that looks superstitious turns out to describe a real and observable behaviour, while the causal story attached to it is wrong.
Because of the porosity, most commercial turquoise is stabilised, impregnated with resin to harden it and fix the colour. Untreated high-grade material is a small fraction of the market. Reconstituted turquoise, dyed howlite and dyed magnesite are all sold as turquoise, and misrepresentation is common.
The matrix, the dark veining, is the surrounding host rock. Spiderweb turquoise, with fine even veining, is valued in some markets and considered a defect in others; Persian taste historically favoured pure unveined blue, and Southwestern American taste often prizes the matrix.
The name is French, pierre turquoise, Turkish stone, not because it came from Turkey but because it reached Europe through Turkish trade from Persian sources.