QUARTZ

QUARTZ (SiO₂)
Group Garnet

Silicon dioxide. Then name may be derived from querkluf tertz(through queretz, quarts, quarz) and old German word apparently refrring to whitish, vein quartz. it is one of then most widely distributed minerals in the eart's crust, some times 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, prsmatic crystals with pyramidal or bipyramidal terminations, which are transparent, coorless, lustrous, and have to cleavage; but quartz may have a smoky appearance or evven 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 separale 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 decoratieve material. Certain varieties of chatoant 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 pink is also used in this way. A granualar, metamorphic rock consisting mainly of quartz (but often containing green mica), known as aventurine, is used as an ornamental material.
Physical properties It has a hardness of 7. The density (2.65 g/cm³) and refractive indices 1.544-1.553 are very constant because of the invariabillity of quartz's chemical composition.
Genesis The largest crystals riginate from the fluids associated with intrusive magmatic phenomena and are fond in pegmatitic, pneumatoltic and hydrothermal depsits. 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.