Encyclopædia Britannica, Ninth Edition/Turquoise

TURQUOISE, a blue or bluish green mineral, valued, when cut and polished, as an ornamental stone. The finest variety occurs in Persia, whence it originally reached western Europe by way of Turkey, and thus came to be called by the Venetians, who imported it, turchesa, and by the French turquoise. It is chemically a hydrated phosphate of aluminium, associated with a variable proportion of hydrated phosphate of copper, to which it owes much of its colour. The green tints of certain varieties appear to be due to admixture with salts of iron. A fine blue Persian turquoise, analysed by Prof. A. H. Church, yielded—alumina 40·19, phosphorus pentoxide 32·86, water 19·34, cupric oxide 5·27, ferrous oxide 2·21, and manganous oxide 0·36. The most valued tint of the turquoise is a delicate blue, inclining slightly to green; in many specimens the green becomes more pronounced with age. Although the turquoise is practically opaque, sections may be ground so thin as to admit of examination by transmitted light. Its microscopic structure was first studied by Prof. Fischer of Freiburg (Baden), afterwards by H. Bücking of Strasburg, and recently by Clarke and Diller. Thin sections are almost colourless. Between crossed Nicols they show either a fibrous texture or a finely-granular aggregate of doubly-refracting particles without definite crystalline outlines. The mineral has never been found crystallized, but occurs as veins, nodules, stalactitic masses, and incrustations. Large pieces are exceedingly rare. The specific gravity of turquoise is about 2·75, and its hardness below 6; it takes a fair polish and exhibits a feeble lustre. It is usually cut en cabochon or with a low convex surface, and in the East is frequently engraved with Persian and Arabic inscriptions, generally passages from the Koran,—the incised characters being in many cases gilt. Such objects are worn as amulets. 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 its owner's health, or even in sympathy with his affections.

Persia is the chief centre of the turquoise trade, where the same mines have been worked for at least eight centuries. The finest stones are found near Nishápúr in Khorásan (see Persia vol. xviii. p. 622). Tavernier, writing in the 17th century, states that the best turquoise, reserved for the sole use of the shah, was obtained from the mine which he describes as the Vieille Roche, while inferior stones were got from the Nouvelle Roche. These terms still survive, all turquoise of fine colour being said in trade to be from the "old rock," and that which is pale, or changes tint on exposure, is from the "new rock." According to a recent report by Consul Benjamin at Teheran the best turquoise is found at Abú Riáh, and all the Khorasan mines are farmed by a few prominent officials, who pay to the shah an annual rent of about £6500. Dr Tietze has lately described specimens of the matrix of the mineral brought to Vienna from Persia by General Schindler. These show that the turquoise occurs, not in clay-slate, as is often stated, but in a porphyritic trachyte or trachytic breccia, and in loose fragments in the neighbouring alluvium. The mineral is also found in Kerman in southern Persia, but the stone is of pale colour, tending to fade, and the mines which yield it are now nearly abandoned. In 1849 Major C. Macdonald found turquoise in Wady Maghara and Wady Sidreh, near Sinai (q.v.), where, according to Mr H. Bauerman, it lines the open joints of a ferruginous sandstone, and is also embedded in small ochreous nodules in the rock itself. The redder the rock, the finer the colour of the associated turquoise. As the colour is liable to fade, the Arabian turquoise has not a good name among jewellers, and the workings were abandoned by Macdonald in 1865. In Wady Maghara there are relics of extensive mining operations, presumably for turquoise, of so early a date that the rock was wrought by flint implements. The early inhabitants of Mexico made much use of this mineral for inlaying obsidian ornaments, and for mosaic work with iron-pyrites. It was probably one of the stones known as chalchihuitl. In 1858 Prof. W. P. Blake called attention to the occurrence of turquoise at Cerillos, about 22 miles south west of Santa Fé, in New Mexico, where mining operations for this mineral were carried on two centuries ago. The turquoise varies in colour from sky-blue to apple-green, and is found as nodules and small veins in a felspathic rock of microgranitic texture, probably of eruptive origin. The mines of Cerillos are no longer worked. A similar green mineral is found at Turquoise Mountain in Cochise county and at Mineral Park, Mohave county, Arizona. It also occurs to a small extent in southern Nevada, where it is found as blue grains disseminated through a sandstone. In Europe, the turquoise is found at Oelsnitz in Saxony and near Jordansmühl in Silesia, occurring at the latter locality in clay-slate. Under the names of callais and callaina Pliny described a green mineral which, if not our turquoise, seems to have been very closely related to it. A bright green mineral, wrought into beads, and found with stone hatchets in ancient graves at Men-er Hrœck (Rock of the Fairy) in Brittany, was described in 1864 by M. Damour, who, seeking to identify it with Pliny's callais, revived this name. Dana afterwards brought the word into harmony with our mineralogical nomenclature by writing it callainite. The mineral in question is a hydrated phosphate of aluminium, apparently identical with Breithaupt's variscite. By many mineralogists the true torquoise is called calaite (vol. xvi. p. 405).

Turquoise is commonly imitated by enamels, but of late some ingenious counterfeits have been made with the same chemical composition as the natural stone. To increase the deception, pieces of ochreous matter are inserted at the back of the artificial turquoise, to imitate the natural matrix. In order to distinguish between the genuine stone and its imitations, Pohl recommends that a splinter should be strongly heated in a platinum capsule, when the true turquoise is reduced to a brownish black powder or a friable mass with a decrepitating sound; the false turquoise does not decrepitate, but either fuses to a glass or is reduced to a frit.

For recent information on the turquoise, see "Das Vorkommen der Türkischei Nischapur in Persien," by Dr E. Tietze, in Verhaudt. D. k. k. geology. Reichsanstalt No. 6, 1884, p. 93: "Mikroskopische Untersuchung des Türkis," by H. Bücking, in Zeitsch. F. Krystallog., vol. ii., 1878, p. 163; "Eine einfache und sichere Unterscheidungsweise der echten Türkise von deren Nachahmungen," in N. Jahrb.f. Mineralogie, 1878, p. 364; "Turqois from New Mexico," by F. W. Clarke and J. S. Diller, in Americ. Journ. Science, Sept. 1886, p. 211; "Revision of Mineral Phosphates, No. iv., Calaite," by A. H. Church, in Chem. News, x. p. 290; and note in Journ. Soc. Arts, xxxii., 1884, p. 1084.