Crystal, Quartz Or Rock Crystal, is a transparent stone, white like a diamond, but much inferior in lustre and hardness. It is used for vases, urns, mirrors, etc. - The ancients knew little of its nature. Pliny speaks of it as hardened petrified water. Several mountains of Europe, and some of Asia, produce rock crystal, Madagascar is said to yield more than all the world beside. Its perfection consists in lustre and transparency : that with straws, dust, clouds, etc., is little valued. It is frequently found hexagonal (or with six sides): the inimitably tine and accurate. It is cut and engraved in the same manner as common glass, of which it was the prototype. F. Francisco Lana, in describing the formation of diamonds, observes, that in the Val Sabbia is a spacious round of a meadow, bare of all herbs, wherein crystals are generated all sexangular. The country people thought them produced from dews, because, being gathered over night, others would arise in a serene and dewy sky. He observed that there was no mark of any mine thereabout, and concluded them produced by the plenty of nitrous streams, which, while it hindered the vegetation of those places, might coagulate the dew falling thereon. Rohault observes, that crystal, diamond, etc must have been originally liquid. Boerhaave takes crystal to be the proper matter or basis of all gems or precious stones ; and that they assume this or that colour from the different admixture of mineral fumes, etc. Rock crystal consists of silica, with a little lime and alumina. There are yellow, green, blue, and violet crystals, called after the precious stones which they resemble, and coloured with iron, as green crystal is called false emerald. Crystal has been erroneously supposed useful in medicine as an astringent, and that it increases milk in nurses, and is an antidote against arsenic.