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For other uses of this word, see Quartz (disambiguation).

Quartz is the most abundant mineral on Earth (about 12% vol.). It has a hexagonal crystal structure made of trigonal-crystallized silica (silicon dioxide, SiO2), with a hardness of 7 on the Mohs scale. The typical shape is a six-sided prism that ends in six-sided pyramids, although these are often distorted, or so massive that only part of the shape is apparent from a mined specimen. Additionally a bed is a common form, particularly for varieties such as amethyst, where the crystals grow up from a matrix and thus only one termination pyramid is present. A quartz geode consists of a hollow pebble (usually an approximately spherical shape), its core lined with a bed of crystals.

Being one of the world's most common minerals, quartz goes by a bewildering array of different names. The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (aggregates of crystals visible only under high magnification). Chalcedony is a generic term for cryptocrystalline quartz. The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline.

Although many of the varietal names historically arose from the colour of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Colour is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. This does not always hold true, however.

Major varieties of quartz:
Variety Description
Chalcedony Any cryptocrystalline quartz, although generally only used for white or lightly coloured material. Otherwise more specific names are used.
Agate Banded Chalcedony, translucent
Onyx Agate where the bands are straight, parallel and consistent in size.
Jasper Opaque chalcedony, impure
Aventurine Translucent chalcedony with small inclusions (usually mica) that shimmer.
Tiger's eye Fibrous quartz, exhibiting chatoyancy.
Rock crystal Clear, colourless.
Amethyst Purple, transparent
Citrine Yellow to reddish orange, greenish yellow
Rose quartz Pink, translucent, may display diasterism
Milk quartz or snow quartz White, translucent to opaque, may display diasterism
Smoky quartz Brown, transparent
Morion Dark-brown, opaque
Carnelian Reddish orange chalcedony, translucent
Not all varieties of quartz are naturally occurring. Prasiolite, an olive coloured material, is produced by heat treatment. Although citrine occurs naturally, the majority is the result of heat-treated amethyst. Carnelian is widely heat-treated to deepen its colour.

Because natural quartz is so often twinned, much quartz used in industry is synthesized. Large, flawless and untwinned crystals are produced in an autoclave via the hydrothermal process: emeralds are also synthesized in this fashion.

Quartz occurs in hydrothermal veins and pegmatites. Well-formed crystals may reach several metres in length and weigh hundreds of kilograms. Erosion of pegmatites may reveal expansive pockets of crystals, known as "cathedrals."

Quartz is a common constituent of granite, sandstone and limestone.

Some quartz crystal structures are piezoelectric and are used as oscillators in electronic devices such as quartz clocks and radios.

A non-crystalline (glass) form of quartz, called fused quartz, can also be produced, distinct from typical window glass.

See also