Quartz, also known as deep quartz or α-quartz, is a mineral with the chemical composition SiO2 and trigonal symmetry. It is the stable form (modification) of silicon dioxide on the earth's surface and the second most common mineral in the earth's crust after feldspar. At a temperature of over 573 °C (under a pressure of 1 bar), low quartz transforms into high quartz by changing its crystal structure.

With a Mohs hardness of 7, quartz is a hard mineral and serves as a reference value on the Friedrich Mohs scale, which goes up to 10 (diamond). It often forms well-developed crystals with a wide variety of shapes and colors, the crystal surfaces of which have a vitreous luster. Quartz is not cleavable, breaks in a conchoidal shape like glass and shows a greasy sheen on the fracture surfaces.

Quartz is one of the most important minerals in industry and is equally important worldwide as a building material and as a raw material for the ceramics, glass and cement industries. Quartz gravel and crushed quartz are raw materials for the extraction of silicon.

In addition, quartz and its colored varieties have been valued as gemstones since time immemorial (see Uses).

Quartz crystals are also produced artificially: Quartz crystals cut from them are used as clock generators in electronic circuits and quartz clocks.

Quartz is occasionally confused with calcite, but can be easily distinguished from it due to its greater hardness, lower birefringence and the reaction of calcite with dilute hydrochloric acid.

Quartz has established itself internationally as a mineral name, with slight, language-specific variations such as quartz in English[4] and French, kvarts in Swedish, quarzo in Italian or кварц (black) in Russian.

According to the 8th and 9th editions of Strunz's Systematics of Minerals, quartz belongs to the mineral class of oxides with a metal-oxygen ratio of 1:2 due to its chemical composition.

In the 8th edition of the mineral systematics, it also gives its name to a group of chemically similar or identical minerals, the quartz group, whose other members are coesite, cristobalite, melanophlogite, mogánite, opal, stishovite and tridymite.

However, the 9th edition of Strunz's mineral systematics subdivides the oxides more finely. Quartz and the related minerals beta-quartz (existence previously only known as a synthesis) coesite, cristobalite, melanophlogite, mogánite, seifertite, opal, stishovite and tridymite are now assigned to the subdivision (chemical compounds) with small cations: Silica family. However, lechatelierite (silica glass), which is also included in the classification, still has a questionable mineral status and is therefore not recognized as an independent mineral by the International Mineralogical Association (IMA).

James Dana's system classifies minerals according to their crystal structure. In quartz, silicon is tetrahedrally surrounded by four oxygen atoms. These SiO4 tetrahedra are linked via their corners to form a three-dimensional framework, and quartz is therefore assigned to the framework silicates in Dana's classification.

Pure quartz is completely transparent and colorless and, if it develops well-formed crystals, is known as Rock crystal is the name given to quartz. Quartz is usually milky cloudy due to microscopic inclusions of liquids and gases (milky quartz) and appears gray when grown into the rock. Transparent to milky cloudy rolled pieces of rock crystal are also known as Rhine pebbles, which originate mainly from the Alpine region and are found in Rhine gravel[8].

Quartz crystals can be coloured differently due to the incorporation of colouring ions (generally Fe3+ or Fe2+), the inclusion of coloured minerals or the effect of ionizing radiation. Based on the color and its cause, the following varieties are distinguished:

Varieties of quartz

Color variations due to foreign ions or irradiation

Amethyst: violet coloration due to the interaction of embedded iron ions and irradiation with gamma rays
Ametrine: rare variety of quartz that shows sectors with amethyst and citrine coloration on a crystal
Citrine: yellow to orange-brown colored quartz (also artificially produced by firing)
Prasiolite (green quartz): leek-green and transparent quartz that rarely occurs naturally and is also produced artificially by firing amethyst or yellowish quartz.
Smoky quartz (morion): colored grey-brown (smoky) to black (morion) by natural or artificial gamma rays
Nickel quartz: coarse green quartz that is colored by nickel.

Color variations due to inclusions

Blue quartz (sapphire quartz): blue, opaque aggregate with embedded crocidolite fibers or dumortierite. Depending on the type of inclusion, blue quartz is also more precisely referred to as crocidolite quartz or dumortierite quartz or dumortierite quartz.
Iron silica: reddish-brown colored quartz due to hematite inclusions
Strawberry quartz is a variety and trade name for a quartz that is irregularly colored pink to red due to reddish-brown hematite inclusions. It is usually more transparent and stronger in color than rose quartz.
Milky quartz: milky-cloudy quartz due to liquid inclusions
Prasem (emerald quartz): leek-green, opaque aggregate that gets its color from inclusions of actinolite.
Rose quartz: cloudy, pink-colored quartz due to dumortierite inclusions, occasionally with asterism due to the inclusion of very fine rutile needles

Microcrystalline SiO2

Microcrystalline quartz refers to massive aggregates of very fine crystalline quartz with crystal sizes in the micrometer range. A distinction is made between three forms:

Chalcedony: microcrystalline, fibrous quartz, fibrously grown along a prismatic surface.
Agate, onyx: microcrystalline fibrous quartz with a parallel-fibered (parabolic) or spherulitic structure
Jasper, carnelian (carnelian, sarder), moss agate, heliotrope, sardonyx, snow quartz
Microquartz: microcrystalline, granular quartz with no recognizable preferred direction of growth
Quartzine: microcrystalline, fibrous quartz, fibrously grown along the base surface (0001) of the hexagonal prism ("length-slow").
Amethyst quartz is an opaque, banded intergrowth of amethyst and milky quartz.

All forms of microcrystalline quartz have a high density of lattice defects and twinning.

Chert and flint are intergrowths of microcrystalline quartz with mogánite in a random, granular structure. Strictly speaking, these are not minerals and mineral varieties, but rocks, which are also summarized under the generic term chert. This sometimes also includes chalcedony and its manifestations as well as amorphous SiO2 (opal).

Other varieties or trade names
Aventurine quartz, hawk's eye, tiger's eye, cat's eye quartz: Quartz with inclusions of platy or fibrous minerals such as fuchsite, rutile, asbestos
The aqua aura often found on the market is not a variety, but usually rock crystal (or another quartz) that has been vaporized with metal (predominantly gold). The result is a transparent, blue-colored crystal, sometimes with a multicolored shimmer.

Brasilite, on the other hand, is the trade name for quartz colored greenish-yellow to pale yellow by firing. The world's first finds of mantle quartz, the tip of which is slightly recessed in the prism, were found in Safiental (Graubünden, Switzerland).