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. Density is 2600kg/m^3. 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 crustal 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:
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."
A amorphous (glass) SiO2, called Lechatelierite, is caused by lightning strikes in sand, distinct from typical window glass that is impure.
The name "quartz" comes from the Greek word Krystallos, meaning "ice". Roman naturalist Pliny the Elder believed quartz to be permanently frozen ice. He supported this idea by saying that quartz is found near glaciers in the Alps and that large quartz crystals were fashioned into spheres to cool the hands. He also knew of ability of quartz to split light into a spectrum. And it was Nicholas Steno's study of Quartz that paved the way for modern crystallography, he discovered that no matter how distorted a quartz crystal the long prism faces always made a perfect 60 degree angle.