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