Turquoise

TURQUOISE, a mineral much used as an ornamental stone for the sake of its blue or bluish-green colour. It is generally held that the name indicates its source as a stone from Turkey, the finest kinds having come from Persia by way of Turkey, whence it was called by the Venetians who imported it turchesa, and by the French turquoise. The old form turkis, used by Tennyson, agrees with the German Turkis. Some authorities have suggested that the word may be a corruption of the Persian name of the stone piruzeh. Turquoise is a crypto-crystalline mineral, occurring in small reniform nodules or as an incrustation, or in thin seams and disseminated grains. Its mode of occurrence suggests its formation by deposition from solution, and indeed it is sometimes found in stalactitic masses. The typical colour is a delicate skyblue, but the blue passes by every transition into green. In some cases the colour deteriorates as the stone becomes dry, and may be seriously affected by exposure to sunlight; whilst with age there is often a tendency to become green, as seen in examples of ancient turquoise. The mineral is always opaque in mass, but generally translucent in thin splinters.

Turquoise takes a fair polish, but the lustre is feeble, and inclines to be waxy; the hardness is nearly 6, the specific gravity between 2-6 and 2-8. Much discussion has arisen as to the chemical composition of turquoise. It is commonly regarded as a hydrous aluminium phosphate having the composition 2A12OYP2CV5H2O or rather Al2HPO.i(OH)4, coloured with a variable proportion of a copper phosphate, or perhaps partly with an iron phosphate. Professor S. L. Penfield, however, has been led by careful analysis of turquoise from Nevada to propose the general formula: [Al(OH)2,Fe(OH)2,Cu(OH),H]3PO4. Hence turquoise may be regarded chemically as derived from orthophosphoric acid by replacement of the hydrogen by the univalent radicles A1(OH)2, &c. An ingenious counterfeit of turquoise has been formed by compressing a precipitate of cupriferous aluminium phosphate. Turquoise is usually cut as an ornamental stone in circular or elliptical form, with a low convex surface.

In the East, where it is used not only for personal ornament but for the decoration of dagger-handles, horse-trappings, &c., the pieces are not unusually of irregular shape; and when worn as amulets the turquoise is often engraved with Oriental inscriptions, generally passages from the Koran, the incised characters being gilt or inlaid with gold wire. The turquoise has always been associated with curious superstitions, the most common being the notion that it changes colour with variations in the state of the owner's health or even in sympathy with his affections. It is commonly held to be a " lucky stone."