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Quartz

A very hard mineral composed of silica, SiO₂, found worldwide in many different types of rocks, including sandstone and granite. Varieties of quartz include agate, chalcedony, chert, flint, opal, and rock crystal.

Quartz, one of the commonest of all rock-forming minerals and one of the most important constituents of the earth's crust. Chemically, it is silicon dioxide. It occurs in crystals of the hexagonal system, commonly having the form of a six-sided prism terminating in a six-sided pyramid; the crystals are often distorted and twins are common.

Quartz may be transparent, translucent, or opaque; it may be colorless or colored. Varieties are classified as crystalline and cryptocrystalline(having a microscopic crystalline structure). Crystalline varieties include ordinary colorless crystallized quartz, or rock crystal(colorless, transparent quartz, used in optical instruments and as a semiprecious gemstone); rose quartz(a pinkish variety of the mineral quartz, used as a gemstone or as an ornamental stone); yellow quartz, sometimes used as imitation topaz; smoky quartz, or cairngorm stone(a transparent or semitransparent brown or gray to nearly black variety of quartz, used as a gemstone); milk-white milky quartz; aventurine quartz(an opaque or semitranslucent brown glass flecked with small metallic particles, often of copper or chromic oxide), which contains scales of hematite or mica; and amethyst.

Varieties of cryptocrystalline quartz, the crystal structure of which can be seen only under the microscope, if at all, are chalcedony(a translucent to transparent milky or grayish quartz with distinctive microscopic crystals arranged in slender fibers in parallel bands), flint, hornstone, and chert(a variety of silica that contains microcrystalline quartz). Colored varieties of chalcedony known by special names are carnelian(pale to deep red or reddish-brown), sard(clear or translucent, deep orange-red to brownish-red), chrysoprase(apple-green), agate(colored bands), onyx(different colors), sardonyx(alternating brown and white bands), and jasper(may be red, yellow, or brown). Clastic quartzes are sand and sandstone.

Material Notes:
Metamorphic rock with the critical mineral quartz (up to100%) and mica. It is formed from the sedimentary rocks quartz-sandstone or conglomerate.

It is found in nearly every geological environment and is at least a component of almost every rock type. It frequently is the primary mineral, >98%. It is also the most varied in terms of varieties, colors and forms. This variety comes about because of the abundance and widespread distribution of quartz. A collector could easily have hundreds of quartz specimens and not have two that are the same due to the many broad catagories.

Quartz is not the only mineral composed of SiO₂. There are no less than eight other known structures that are composed of SiO₂. These other substances and quartz are polymorphs of silicon dioxide and belong to an informal group called the Quartz Group or Silica Group. All members of this group, except quartz, are uncommon to extemely rare on the surface of the earth and are stable only under high temperatures and high pressures or both. These minerals have their own unique structures although they share the same chemistry, hence the term polymorph, which means many forms.

Quartz has a unique structure. Actually, there is another mineral that shares quartz's structure, and it is not even a silicate. It is a rare phosphate named berlinite, AlPO₄, that is isostructural with quartz. The structure of quartz involves corkscrewing (helix) chains of silicon tetrahedrons. The corkscrew takes four tetrahedrons in order to repeat itself, or three turns. Each tetrahedron is essentially rotated 120 degrees. The chains are aligned along the C axis of the crystal and interconnected to two other chains at each tetrahedron making quartz a true tectosilicate. This structure is not like the structure of the chain silicates or inosilicates whose silicate tetrahedronal chains are not directly connected to each other. The structure of quartz helps explain many of its physical attributes.

Quartz is a fun mineral to collect. Its abundance on the Earth's surface is incredible and produces some wonderful varieties that don't even look like the same mineral. A collector must always be up on the many varieties of quartz and it sometimes embarrasses a collector to have collected too many specimens of such a common mineral. But nearly all collectors concede that you can never really have enough quartz specimens.

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