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.
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 forma tion by deposition from solution, and indeed it is sometimes found in stalactitic masses. The typical colour is a delicate sky-blue, 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 or rather coloured with a variable proportion of a copper phosphate, or perhaps partly with an iron phosphate. S. L. Pen field (1900), however, was led by careful analysis of turquoise from Nevada to propose the general formula : An analysis of minute crystals from Virginia gave the formula 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, etc., the pieces are usually of ir regular shape. In Persia, where the finest turquoise is found, the mines have been worked for at least eight centuries. The principal
locality is near Nishapur, in the province of Khorasan. Here the turquoise occurs in narrow seams in a brecciated trachyte-por phyry. It is found also in some other localities in Persia and in Turkestan. In the Sinai Peninsula relics of extensive ancient min ing operations for turquoise show that the rock was at one time worked with flint implements. The mineral here occurs as nodules in a red sandstone and as an incrustation on the joint-faces.
In ancient Mexico much use was made of turquoise as an inlay for mosaic work, with obsidian, malachite, shell and iron pyrites. Relics of extensive workings are found in the mountains of Los Cerillos near Santa Fe in New Mexico, where mining for tur quoise is now actively carried on. One of the hills in which old workings occur has been called Mt. Chalchihuitl, since it is be lieved that the turquoise was known by the name chalchihuitl, which in some places was applied also to jade. The matrix at Los Cerillos is an altered augite-andesite, in which the turquoise occurs in thin veins and in small nodules in patches of kaolin.
Turquoise is sometimes termed by mineralogists callaite, since it is believed to be the callais of Pliny. The name callainite was suggested by J. D. Dana for a bright green mineral which was found in the form of beads, with stone hatchets, in ancient graves near Mane-er-H'roek (Rock of the Fairy), near Locmariaquer in Brittany, and which A. Damour sought to identify with Pliny's callais. The. mineral seems to be identical with variscite (q.v.).
"Bone-turquoise" or odontolite, also known as "occidental tur quoise," is merely fossil bone or ivory coloured by iron phosphate (vivianite) or perhaps stained in some cases by cupriferous solu tions, and is readily distinguished from true turquoise by showing organic structure under the microscope.