The most abundant and common mineral, consisting of crystalline silica (silicon dioxide, SiO2), crystallizing in the trigonal system. It has a hardness of 7 on the Mohs' scale. Well-formed crystals of quartz are six-sided prisms terminating in six-sided pyramids. Quartz is ordinarily colourless and transparent, in which form it is known as rock crystal. Coloured varieties, a number of which are used as gemstones, include amethyst, citrine quartz (yellow), rose quartz (pink), milk quartz (white), smoky quartz (grey-brown), chalcedony, agate, and jasper. Quartz occurs in many rocks, especially igneous rocks such as granite and quartzite (of which it is the chief constituent), metamorphic rocks such as gneisses and schists, and sedimentary rocks such as sandstone and limestone. The mineral is piezoelectric and is used in oscillators. It is also used in optical instruments and in glass, glaze, and abrasives.