Quartz (Tectosilicates)

Quartz silicon oxide SiO2
large geode of purplish amethyst crystals			detail of a similar geode highlighting a single amethyst crystal
single large quartz crystal showing the rare trigonal trapezohedron face			detail of the trigonal trapezohedron face of same specimen
detail of a variety called 'Herkimer diamond' showing adamantine luster; these exceptionally clear crystals form in vugs in limestones

Diagnostic properties:

Color: usually colorless or white, but impurities may cause it to be any color.
Streak: white.
Hardness: 7.
Specific Gravity: 2.65.
Luster: vitreous; in some specimens, greasy, splendent.
Cleavage: none.
Fracture: conchoidal.
Transparency: transparent to translucent.
Other properties: strongly piezoelectric and pyroelectric.
Occurrence: very common.
Crystal system: hexagonal.
Uses: in pressure gauges, oscillators, resonators and wave stabilizers (because of its piezoelectric property); in polarimeters (because of its optical properties); in heat-lamps, prisms and spectrographic lenses (because of its transparency to ultraviolet rays); also in the manufacture of glass, paints, abrasives, refractories; semi-precious varieties are used as gems and ornamental stones.
Name: from a German word.
Varieties:
- Rock crystal - colorless, commonly in distinct crystals.
- Amethyst - violet.
- Rose quartz - rose-red or pink.
- Smoky quartz - smoky yellow to brown to almost black.
- Citrine - light yellow.
- Milky quartz - milky white.
- Tiger eye - with yellow parallel fibrous quartz pseudomorphic after asbestiform amphibole.
- Rutilated quartz - with inclusions of fine needles of rutile.
- Aventurine quartz - with inclusions of scales of colored minerals like hematite (red) or chromium mica (green).
- Chalcedony - microcrystalline quartz, usually brown or gray.
- Carnelian - red chalcedony.
- Chrysoprase - apple-green chalcedony.
- Agate - chalcedony with alternating layers of different colors and porosities.
- Onyx - chalcedony with layers arranged in parallel planes.
- Sardonyx - onyx with sard alternating with white or black layers.
- Heliotrope or bloodstone - green chalcedony with red specks of jasper in it.
- Flint - dark microcrystalline quartz.
- Chert - lighter-colored microcrystalline quartz; there is no sharp distinction between flint and chert.
- Jasper - granular microcrystalline quartz usually colored red by hematite.
- Prase - dull green microcrystalline quartz; similar to jasper.

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