Quartz

Quartz is silicon dioxide, SiO₂, and one of the most common minerals of the earth's crust. It grows as six-sided prisms with pointed terminations and, with a hardness of 7, scratches glass. Pure quartz is colorless rock crystal; traces of other elements and natural radiation produce its colored varieties, among them amethyst, smoky quartz and citrine. Collectors look for form as much as color: scepters, phantoms, Japan-law twins joined at nearly a right angle, and the twisted gwindels of the Alps. Sources include the Alpine clefts of Switzerland and Austria, the Ouachita Mountains of Arkansas, the pegmatites of Minas Gerais in Brazil and of Gilgit-Baltistan in Pakistan, and the ore veins of Cavnic and Dalnegorsk, where it grows with sphalerite and galena.