Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2.
There are many different varieties of quartz, several of which are semi-precious gemstones. Throughout the world, varieties of quartz have been since antiquity the most commonly used minerals in the making of jewelry and hardstone carvings.
Some forms of Quartz, especially the gemstone forms, have their color enhanced. Almost all forms of the yellow-brown variety Citrine are in fact heat treated.
Much Amethyst is also heat treated to intensify color, and a green transparent form known as "Green Amethyst" or "Prasiolite" is formed by heat treating certain types of Amethyst.
There is also a transparent sky blue form of Quartz crystals, as well as a wildly iridescent type that are synthetically colored by irradiation of gold. In some localities, Hematite forms a thin red or brown layer internally in the Quartz crystal, giving it a natural bright red to brown coloring, and sometimes even a mild natural iridescence.
- Quartz is an important mineral with numerous uses. Sand, which is composed of tiny Quartz pebbles, is the primary ingredient for the manufacture of glass.
- It is used as oscillators in radios, watches, and pressure gauges, and in the study of optics. Quartz is also used as an abrasive for sandblasting, grinding glass, and cutting soft stones. It is also essential in the computer industry, as the important silicon semiconductors are made from Quartz.
- In addition to all the practical uses, Quartz is essential to the gem trade. Many varieties are faceted as gems. Amethyst and Citrine are the most well-known gem varieties. Rose Quartz, Smoky Quartz, Rock Crystal, and Aventurine are also cut or polished into gems. Small colorless Quartz crystals are worn by some as pendants for good luck.
- Quartz is also a very popular among collectors. Certain collectors specialize their collection entirely on Quartz alone.