Silicon dioxide. The name may be derived from querkluftertz. (through querertz, quarts, quarz) an old German word apparently referring to whitish, vein quartz. lt is one of the most widely distributed minerals in the earth's crust, sometimes found as elegant crystals whose luster, hardness, and watery transparency or, conversely, pleasing colors have long been a source of fascination, causing it to be widely employed as a gem or ornamental material.

Crystal system Trigonal.
Appearance The most typical form consists of hexagonal, prismatic crystals with pyramidal or bipyramidal terminations, which are transparent, colorless, lustrous, and have no cleavage; but quartz may have a smoky appearance or even be black, yellow, brownish yellow, violet. or pink. Massive, white, milky vein quartz is very common. A. microcrystalline variety of quartz, found as compact, massive concretions, is called chalcedony; this has separate colors, sometimes with distinct color banding, in which case it is known as agate, sard, cornelian, plasma, etc depending on the color, a whole series of names having been evolved during its long history as a decorative material. Certain varieties of chatoyant quartz are also used as gems and ornaments (cat's-eye, tiger‘s-eye, hawk's-eye, bull's-eye). Of the semiopaque, massive varieties, the pinks also used in this way. A granular, metamorphic rock .;OnSiSting mainly of quartz (but often containing greenmica), known as aventurine, is used as an ornamental material.

Physical properties It has a hardness of 7. The density (2.65 g/cmef) and refractive indices (nw 1.544, ne 1.563) are very constant because of the invariability of quartz's chemical composition.

Genesis The largest crystals originate from the fluids associated with intrusive magmatic phenomena and are found in pegmatitic, pneumatolytic and hydrothermal deposits. Quartz also occurs in sedimentary and metamorphic environments, but as very small crystals. The microcrystalline varieties are often associated with hydrothermal processes, even under the sea.

Occurrence Quartz is extremely widespread, the most famous localities for magnificent, large crystals being the French Alps, the St. Gotthard massif in the Swiss Alps, the United States, Brazil, and Madagascar.