Rock Crystal—This is quartz in its purest form. Typical rock crystal is perfectly transparent and colorless, but the mineral is often more or less clouded and opaque. By the ancients it was supposed to be petrified ice, and hence the Greeks applied to it their word for ice, from which we get our word crystal. The belief in its ice origin survived to a comparatively late period, for in 1676 Robert Boyle opposed the idea, stating that the quartz could not be ice, first because it was two and a half times as heavy as water, and second because it was found in tropical countries. The belief of the ancients probably came largely from the fact that the quartz they knew was obtained from the peaks of the Alps. They reasoned that it was ice that was frozen so hard that it would never melt. Fortunately our present knowledge of chemistry prevents us from any longer confounding the two substances, for we know Quartz is oxide of silicon while water is oxide of hydrogen. Quartz in the form of rock crystal occurs in all parts of the globe, and for the most part in well-formed crystals. These crystals are usually six-sided, and usually have the form of a prism capped by a pyramid. Hot Springs, Arkansas, and Little Falls, New York, are the best known localities in our own country for this form of crystallized quartz. The Little Falls crystals are exceptionally brilliant and well formed. From this locality and others the material, cut or uncut, is often known as diamonds, and sold as such. Such stones can, of course, be easily distinguished from true diamond, for while they will scratch glass, their hardness is much below that of the king of gems and they utterly lack the internal fire of the latter. Rock crystal occurring in large, clear masses is often cut into ornamental and useful objects such as seals and paperweights, and especially into balls. The latter industry flourishes especially in Japan, and perfectly clear quartz balls six inches in diameter are made there. Rock crystal is also used extensively to cut into eyeglasses and spectacles in place of glass, some considering it less detrimental to the eyes than glass. It is also occasionally used for mirrors, it being superior to glass for this purpose, in that it does not detract from the rosiness of the complexion. Among the ancients rock crystal was much more highly prized than among us, as it answered them many of the purposes for which we now find glass more suitable and cheaper. Wine glasses were made from it, though of course at great cost, a thousand dollars being considered a small price for one. Lenses of rock crystal were used to concentrate the rays of the sun for cauterizing wounds and also to light fires, especially sacrificial ones. Roman ladies were also accustomed to carry balls of rock crystal in their hands in summer for the sake of the coolness they afforded. The ladies of Japan are said to do the same at the present day. [Illustration: QUARTZ (crystalline.)]