The colored varieties of the silica minerals
Quartz, when chemically pure SiO_2, is colorless and transparent from about 145 nm in the ultraviolet to about 2700 nm in the infrared when in mm to cm thickness. Natural quartz has three important colored varieties: amethyst (violet), citrine (yellow to brown) and smoky (dark smoky quartz is sometimes called morion). A fourth variety, prasiolite (green) also exists. Each of these varieties exhibits either a substitutional or interstitial component other than Si. In addition to chemical substitutions, development of color in some of these varieties requires exposure to ionizing radiation or heat. Each of these varieties can be produced through heat-treatment or irradiation of natural quartz of a different color and can also be produced by direct synthesis or through synthesis followed by irradiation. Other colored varieties of natural quartz (rose, blue, chrysoprase, jasper, etc.) are, in fact, mixtures of quartz and other phases (although there may be a type of rose quartz from substitutional causes alone).
© 1994 Mineralogical Society of America. Support for the original research which is part of this chapter was provided by the White Rose Foundation, whose interest in the scientific basis of the origin of color in minerals is highly appreciated. Assistance with the translation of Russian articles for this chapter was provided by Ruslan Mendybaev. Helpful discussions about quartz with Kurt Nassau, John Weil and Paulo Vasconcelos are particularly appreciated as are the numerous museums, miners and other donors who provided samples of quartz minerals over the years.