Quartz, a glasslike mineral composed of silicon and oxygen. Forming about 12 per cent of the earth's crust, quartz is more abundant than any other mineral except feldspar. Sand and sandstone are chiefly quartz. Quartz is found also in igneous rocks such as pegmatite and granite. It exists in sizes ranging from tiny specks to pieces weighing a ton or more. Pure quartz, often called rock crystal, is colorless. Most quartz, however, is colored by impurities. Quartz has a high melting point, is not readily affected by moisture or by temperature changes, and does not combine readily with chemicals. It is the hardest of common minerals.
There are two basic kinds of quartz: (1) crystalline, consisting of crystals large enough to be seen with the unaided eye; and (2) cryptocrystalline, commonly called chalcedony, consisting of crystals so fine that they can be seen only with the aid of a microscope. Among crystalline varieties are amethyst, cairngorm (smoky quartz), rock crystal, and rose quartz. Chalcedony quartzes include agate, bloodstone, carnelian, flint, jasper, onyx, and sardonyx.
Quartz crystals have piezoelectric properties, meaning that they are capable of changing a mechanical force into electricity, or an electric current into a mechanical force. A slice, or wafer, of quartz crystal will generate an electric current when it is subjected to pressure. Conversely, a wafer connected in an alternating electric circuit will expand and contract, or oscillate, at a fixed frequency. This frequency depends on the thickness of the wafer. Thin wafers oscillate at higher frequencies than thick ones.
Quartz crystal wafers are used to control the frequencies of oscillating electric circuits in quartz watches and radio transmitters. They are also used to convert electric signals into sound waves in devices such as sonar and ultrasonic generators. In such devices as hearing aids and submarine detectors, quartz crystals convert weak sound waves into electric current, which is then amplified and reconverted into sound waves. Some phonographs use quartz crystals to convert vibrations of the stylus (needle) into electric impulses. For electronic use, a quartz crystal must be flawless. Most of the crystals now used in electronics are synthetic.
Rock crystal is used to make lenses and prisms for optical instruments. Quartz is also used to make glass, mortar, grindstones, sandpaper, and cleaning compounds. It is used as a flux in the smelting of iron and copper ores, and as a glaze for porcelain. Such varieties of quartz as agate, amethyst, onyx, and sardonyx are used as gems.
Chemical formula: SiO2. Specific gravity: 2.59-2.66. Hardness: 7.