Turquoise
A sky-blue copper phosphate valued for millennia — soft, porous and often stabilised.
Also known as: Copper aluminium phosphate
intermediate Arid-region copper deposits
What it is
Turquoise is a hydrated copper aluminium phosphate, CuAl₆(PO₄)₄(OH)₈·4H₂O, whose blue comes from copper and whose green tinge comes from iron. It is soft and porous at 5 to 6 on the Mohs scale, so it readily absorbs oils and can change colour with wear; much commercial turquoise is therefore stabilised with resin to harden it and protect the colour.
Prized for over 6,000 years by Persian, Egyptian and Native American cultures, the finest is an even robin's-egg blue, sometimes veined with brown or black "matrix" from the host rock. Historic sources include Iran (Nishapur), the American Southwest and China. Buyers should ask whether a stone is natural, stabilised or reconstituted, as treatment strongly affects value.