Figure 1. Quartz forms long hexagonal crystals with pointed ends.
Figure 2. Quartzite. Image credit: Mineral Information Institute.
Quartz is a hard mineral that is a crystalline form of silica (silicon dioxide, SiO2). It is colorless when pure – a form also known as "rock crystal".
Quartz is a constituent of many rocks, especially igneous rocks such as granite and quartzite, metamorphic rocks such as gneisses and schists, and sedimentary rocks such as sandstone and limestone. It is the second most abundant mineral on Earth after feldspar.
Quartz crystallizes in the trigonal system. A well-formed quartz crystal
consists of a six-sided prism that terminates in six-sided pyramids (see Figure 1). Quartz
is usually colorless and transparent, in which form it is called rock
crystal. Colored varieties,a number of which are used as gemstones,
include amethyst, citrine
(yellow), rose quartz (pink), milk quartz (white), smoky quartz (brown), agate, jasper, onyx, carnelian,
and chalcedony. It has a hardness of
7 on Mohs scale; relative density 2.65.
A quartz crystal produces electricity through the piezoelectric effect when a mechanical stress is applied to it. This has led to the use of quartz for making oscillators for clocks, radios, and radar instruments. Quartz is also used in optical instruments and in glass, glaze, and abrasives.
Stishovite is a dense, high-pressure phase of quartz that has so far been identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters.
Quartzite is a hard, massive metamorphic rock resulting from sandstone in which the quartz grains have recrystallized (see Figure 2). It is typically colored white, light gray, or yellow, but it also comes in green, blue, purple, and black varieties that contain other minerals. Tough and resistant to weathering it is used in road metal.