The term quartz is applied to all silicious minerals, such as rock crystal, agate, silex, and jasper, which are infusible under the blow-pipe, insoluble in acids, and scratch glass. But it is more generally applied to the hyaline species, or rock crystal. Its limpid clearness has at all times caused it to be held in request, as suitable for vases, tazze, and other such objects. The ancients both cut and engraved it; but especially since the Renaissance it has been largely applied to ornamental purposes, being carved into vases, cups, comfit-boxes, etc, frequently embellished with goldsmiths' work and gems.

Perfect limpidity is the principal merit of crystal, which is, however, occasionally found with more or less decided tints. Not to mention the amethyst, a species quite apart, there is the brownish orange quartz of Brazil, which is very effective, while the smoky quartz, known as brown crystal, or Alencon diamond, sometimes comes from China fashioned into objects of considerable size.

The "girasol" quartz, an extremely rare variety, has a milky ground emitting lovely roseate and delicate blue flashes. It comes from Siberia.