ROCK-CRYSTAL, a colourless and transparent variety of quartz, used as an ornamental stone. It usually occurs as crystals lining cavities in quartz-veins, which often run through granite, gneiss and crystalline schists. The limpidity of the crystal, its coldness to the touch and its common occurrence in rocks among Alpine glaciers, led to the ancient belief that it was a kind of congealed water, whence the name crystal, from Gr. KpuaraXXos (ice). In the Swiss Alps the "Strahler," or crystalgatherer, searches the rocks at much personal risk, and is often led to. a drusy cavity by tracing narrow veins, or strings, of quartz on the mountain-side. A remarkable druse, or Krystalkeller, discovered at Zinkenstock in the Bernese Oberland, in 1719, yielded about 20 tons of crystal, a single specimen weighing 8 cwt. The famous discovery of the Galenstock, in 1867, furnished magnificent crystals, but they were dark brown or smoky quartz. La Gardette, near Le Bourg d'Oisans, in the Alps of Dauphine, is a notable locality for fine specimens of rock-crystal. The Alps and India probably furnished the ancients with their supplies.
Rock-crystal has been used for ornamental purposes since the Mycenean period. By the Romans under the Empire it was highly valued, and carved into vases and goblets, in some cases elaborately engraved. Lenses or globes were used for kindling the sacred vestal fire and for cauterizing the flesh, whilst ladies carried balls of crystal in order to cool their hands during the heat of summer. The artists of the Early Renaissance greatly favoured the use of rock-crystal, and executed beautiful carvings in this material. In modern times the use of rock-crystal has been largely superseded by that of glass, and it is notable that flint-glass is known in France as "cr y stal," probably from its resemblance to limpid quartz, or perhaps from the fact that powdered rock-crystal has been used as a source of silica in the manufacture of the finest glass. Rockcrystal is still cut as a faceted stone for personal decoration, but though not without brilliancy it lacks the "fire" of many gemstones. It is often known locally by such names as Bristol diamond, Cornish diamond, Isle of Wight diamond, Briancon diamond, Marmaros diamond, Lake George diamond, &c. Rock-crystal is also carved into seals, paper-weights and other trivial objects, and into spheres for divination by crystalgazing, Japanese balls being specially noteworthy. In Japan the crystal has been obtained for centuries from the granitic districts around Kimpu-san, in the province of Kai. Probably the most valuable application of rock-crystal is for spectacle lenses, which in consequence of their hardness are not readily abraded by use. They should be cut at right angles to the optic axis, or axis of the prism.
The "pebble" for lenses is found loose in the soil in many parts of the provinces of Goyaz, Sao Paulo and Minas Geraes in Brazil. Much of the material for spectacles comes also from Madagascar, where large crystals of clear quartz are found in the beds of certain streams, especially in the N.E. part of the island, having probably been derived from quartz-veins in the gneiss and pegmatite. In India rock-crystal has been worked at many localities, and the loot of the palace of Delhi yielded marvellous ornaments carved in this material. At the present day it is cut and polished at Vellum in the Tanjore district in Madras, and is known as Vellum stone. Among the numerous localities in the United States which yield rock-crystal mention may be made of those in Herkimer Co., New York State, whence the Lake George crystals are obtained; and it is notable that some of the Herkimer quartz encloses bituminous matter. Mokelumne Hill, Calaveras Co., California, has furnished some remarkable rock-crystal. In Europe the localities are very numerous, the most important being those in the Alps. Very fine crystals remarkable for pellucidity though not of large size occur in cavities in the statuary marble of Carrara; and remarkably hollowed crystals are known from Porretta near Bologna in Italy. The finest rock-crystal in Great Britain occurs at Tintagel and the Delabole slate quarry in N. Cornwall; and at Snowdon in N. Wales. (F. W. R.*)
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