The New Student's Reference Work/Quartz

Quartz (käwrts), the name of the commonest mineral substance of nature. Chemically, quartz is silicon oxide (SiO$2$). Quartz occurs in nature in the crystalline and the noncrystalline forms. When crystalline, it occurs in six-sided prisms terminated by pyramids. Well-developed crystals are most commonly found in veins or open spaces in rocks. By far the larger part of quartz, even that which is really crystalline, does not occur in the form of prisms, but in the form of sand grains, pebbles etc., which themselves were derived from crystals. Quartz is one of the constituent minerals of most light-colored igneous and metamorphic rocks; is the chief constituent of most sandstone; and is an important constituent of most other sorts of sedimentary rock, except limestone. It is the hardest of the common minerals. Pure quartz is transparent, but it often contains impurities which give it distinctive and sometimes beautiful colors. Several varieties of quartz constitute gems, some of them of great value. Here belong amethyst, a transparent, purple variety of quartz; agate, the banded variety of quartz from which cameos are cut; chalcedony, a translucent variety; smoky quartz; cat's eye quartz; jasper, a red, opaque variety; various so-called topazes (not the true topaz); rose quartz, a pinkish or rose-tinted variety; and rock crystal, or crystals of pure quartz. Flint and chert are impure varieties of quartz.