quarz.] The common name of silicon oxide or silica, the most abundant of all minerals, being one of the constituents of granite, gneiss, mica slate, etc. It forms quartz rock and sandstone, and makes most of the sand of the seashore. It occurs massive, crystallized, granular, and in other forms. The primary form of the crystal is a rhomboid; but it is generally met with in hexagonal prisms, terminated by hexagonal pyramids. When crystallized and pure, it is called rock-crystal, and is transparent and colorless. Quartz is so hard that it will scratch glass and strike fire against steel. It comprises numerous varieties, many of which are colored by different substances-as purple quartz, or amethyst, rose quartz, yellow quartz, chalcedony, agate, carnelian, bloodstone, jaspar, sard, onyx, cat's-eye, etc. Quartz is used in the manufacture of glass, and of porcelain and other kinds of pottery ; also as a flux in the smelting of several kinds of, ores. Gold often occurs in quartz veins, and quartz-crushing machines are used to extract the gold ore.