Quartz is an abundant mineral in the Earth's continental crust. 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. Quartz belongs to the trigonal crystal system. The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. In nature quartz crystals are often twinned, distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal...'
Contributions by Vsmith, Xenoalien, and Zzorse.