Turquoise is a phosphate and hydrate of alumina, associated with a hydrated phosphate of copper; it always contains small quantities of phosphate of iron and manganese. A fine Persian specimen contained in 100 parts:
Phosphorus pentoxide | 32·8 | Water | 19·3 |
Alumina | 40·2 | Copper oxide | 5·3 |
Iron and manganese oxides | 2·5 |
Turquoises of considerable size and occasionally of good colour are met with having Persian and Arabic inscriptions or ornamental designs engraved upon them. The hollows of the designs are sometimes gilt, sometimes inlaid with gold wires.
The distinction between turquoises de la vieille roche and those de la nouvelle roche has a real existence. The former, chiefly obtained from the Nishapur district, are superior in quality of colour, even when the latter belong to the same species, and are not, as is often the case, identical with odontolite, or bone turquoise, or with variscite.
The turquoises from New Mexico vary much in colour; and if originally of a fine blue do not invariably preserve their hue. Some of them contain interspersed particles of quartz. No less than 7 per cent. of copper oxide occurs in some of the stones from the Burro Mounts, Grant County.
Callainite.
There are three minerals passing under the name of turquoise. The true turquoise is the callaite of mineralogists; then there is a fossil turquoise or odontolite; and lastly we have a pale bluish-green stone which has been described under the names callais, variscite, and callainite, and which presents a near relationship to the true turquoise. Its hardness is 4; its specific gravity 2·55; and its percentage composition :
Alumina | 32·4 | Phosphorus pentoxide | 44·9 |
Water | 22·7 |
There is another blue mineral related chemically to turquoise