ROCK CRYSTAL, a popular and partly also a scientific name for the finest and purest quartz (q.v.), seldom applied, however, to small crystals which are mere six-sided pyra mids, but more generally to those in which the six-sided prism is well developed. The name is sometimes limited to colorless and perfectly transparent quartz, but is also more rarely extended to that which is violet or amethystine (amethyst, q.v.), red (Bohemian ruby or Silesian ruby), wine-yellow (citrin or gold topaz), brown or smoky ($rnoke quartz, Cairngorm stone), etc. The beauty of specimens of rock crystal is sometimes very great. The crystals are sometimes slender, crossing and penetrating each other in exquisite groups. They sometimes inclose other substances, which are beautifully seen through the transparent rock crystal, as slender hair-like or needle-like crystals of hornblende, ashestus, oxide of iron, rutile or oxide of titanium, oxide of manganese, etc., and such
specimens are known by various fanciful names, as Thetis's hair-stone, Venus's hair-stone, Venus's pencils, Cupid's net, Cupid's arrows, etc.; and sometimes the inclosed substances are small spangles of iron-glance, or crystals of iron pyrites, or native silver in fern-like leaves, or spangles of gold. Very large crystals of perfectly pure rock crystal are some Times found. One found in the Alps, and which was among the treasures carried from Italy by the French in 1797, is 3 ft. in diameter, and weighs S cwt. Rock crystal was prized by the ancients, and was used by them, as it ,still is, for vases, cups, seals, etc. An important modern use of it is for lenses of spectacles, etc., its hardness rendering it much less liable to be scratched than glass. Lenses of rock crystal are often called pebbly lenses.,