Tourmaline, a name applied to a group of rhombohedral double silicates, composed of silica, fluorine, boric acid, alumina, manganic, ferric, and ferrous oxides, magnesia, lime, soda, potash, lithia, and sometimes phosphoric acid. Rammelsberg divides them into magnesium, magnesium-iron, iron, iron-manganese, and manganese tourmalines, the last two varieties alone containing lithia. The sesquioxides are alumina and ferric and manganic oxides. The color of tourmalines varies with their composition; the red, called rubellite, are manganese tourmalines, containing lithium and manganese, with little or no iron; the violet blue (called indicolite) and green are iron-manganese tourmalines; and the black, which are schorl, are either iron or magnesium-iron tourmalines. White or colorless tourmalines, which arc rare, are called achroite. Sometimes the crystals are red at one extremity and green at the other, or green internally and red externally, or vice versa. Tourmaline is usually found in granite, gneiss, and syenite, in mica, chloritic, and talcose schists, in dolomite, granular limestone, and sometimes sandstone near dikes of igneous rocks (Dana). Rubellite and green tourmaline are found at Yekaterinburg in the Ural mountains; pink crystals in the island of Elba; pale yellowish brown in Carinthia; white in the St. Gothard mountains, the Ural, and Elba. In Massachusetts, at Chesterfield, are red, green, and blue tourmalines, in a granite vein with albite; and at Goshen the blue occurs in great perfection.

At Grafton and Orford, 1ST. II., Brattleboro, Vt., and Monroe, Conn., specimens of tourmaline of various colors occur in steatite, mica slate, and other rocks. Tourmalines are found in New York, at Crown Point, in fine brown crystals, and in St. Lawrence, Jefferson, Essex, and other counties; also in other states, in numerous localities. In California black crystals, 6 to 8 in. in diameter, occur in feldspar veins in the mountains between San Diego and the Colorado desert. In Canada, superb greenish yellow crystals an inch in diameter occur in limestone at Grand Calumet island. In the town of Paris, Maine, in one of the spurs of "Streaked mountain " called by the mineralogists Mt. Mica, several deposits of beautiful green and red tourmalines of perfect forms were found in 1820 by Elijah L. Hamlin and Ezekiel Jones. Many specimens were sent to various parts of Europe; and some fine ones obtained from Vander Null, an antiquary, are believed to be in the museum at Vienna. - Tourmalines are not often used in jewelry, although fine rubellites form beautiful gems, and bear a high price. In the grand duke's collection at Florence there was a specimen 11 in. square, with four erect green tourmalines and one prostrate, 4, 2, and 2¼ in. long and f in. to 1 in. thick.

A magnificent group of pink tourmalines nearly a foot square was given by the king of Burmah to Col. Sykes, while commissioner to his court. The tourmaline appears to have been first brought to Europe from Ceylon by the Dutch about the end of the 17th century, and was exhibited as a curiosity on account of its pyro-electric properties, whence it was called aschentrecker (Ger. Aschenzieher). The tourmaline is a double-refracting crystal, but has the peculiar property of polarizing light. It has not the power like Iceland spar of separating and transmitting both the ordinary and the extraordinary ray; but when the plate is cut with its faces parallel to the optic axis of the crystal, and exposed to a ray of light, the ordinary ray passes through, while the extraordinary ray is absorbed. (See Light, vol. x., pp. 446 and 449, and Tiieemo-Electeicity.) - See "Diamonds and Precious Stones," by Harry Emanuel (London, 1867; New York 1873); "The Tourmaline," by A. C. Hamlin, M. D. (Boston, 1873); and Dana's " Mineralogy".