Quartz

Quartz is a hard mineral composed of silica. Its atoms are linked in a continuous framework of SiO₄ silicon–oxygen tetrahedra, with each oxygen atom being shared between two tetrahedra, giving an overall chemical formula of SiO₂. Therefore, quartz is classified structurally as a framework silicate mineral and compositionally as an oxide mineral. Quartz is the second most common mineral or mineral group in Earth's lithosphere, comprising about 12% by mass.
Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral. The transformation from α-quartz to β-quartz takes place abruptly at. Since the transformation is accompanied by a significant change in volume, it can easily induce microfracturing of ceramics or rocks passing through this temperature threshold.
There are many different varieties of quartz, several of which are classified as gemstones. Since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Europe and Asia.
Quartz is the mineral defining the value of 7 on the Mohs scale of hardness, a qualitative scratch method for determining the hardness of a material to abrasion.

Etymology

The word quartz is derived from the German word Quarz, which had the same form in the first half of the 14th century in Middle High German and in East Central German and which came from the Polish dialect term kwardy, which corresponds to the Czech term tvrdý. Some sources, however, attribute the word's origin to the Saxon word Querkluftertz, meaning cross-vein ore.
The Ancient Greeks referred to quartz as κρύσταλλος meaning "crystal", derived from the Ancient Greek κρύος meaning "icy cold", because some philosophers believed the mineral to be a form of supercooled ice. Today, the term rock crystal is sometimes used as an alternative name for transparent, coarsely crystalline quartz.

Early studies

Roman naturalist Pliny the Elder believed quartz to be ice, permanently frozen after great lengths of time. He supported this idea by saying that quartz is found near glaciers in the Alps, but in warm climates. This idea persisted until at least the 17th century.
In the 17th century, Nicolas Steno's study of quartz paved the way for modern crystallography. He discovered that, regardless of a quartz crystal's size or shape, its long prism faces always meet at a perfect 60° angle, thereby establishing the law of constancy of interfacial angles.

Crystal habit and structure

Quartz can form as two distinct polymorphs depending on the temperature and pressure: α-quartz and β-quartz. α-quartz crystallizes in the trigonal crystal system, while β-quartz has greater symmetry and crystallizes in the hexagonal crystal system. The transition from α-quartz to β-quartz occurs abruptly at at ambient pressure; the transition temperature is greater at higher pressures. β-quartz is unstable at room temperature; therefore, all quartz at room temperature is α-quartz regardless of which polymorph it formed as.
Both polymorphs of quartz can occur in two different space groups depending on the chirality. Above the transition temperature, α-quartz in P3₁21 becomes β-quartz in P6₄22, and α-quartz in P3₂21 becomes β-quartz in P6₂22.
These space groups are truly chiral. Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks. The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without a change in the way they are linked. However, there is a significant change in volume during this transition, and this can result in significant microfracturing in ceramics during firing, in ornamental stone after a fire and in rocks of the Earth's crust exposed to high temperatures, thereby damaging materials containing quartz and degrading their physical and mechanical properties.
The ideal crystal shape for quartz is a six-sided prism terminating with six-sided pyramid-like rhombohedrons at each end. In nature, quartz crystals are often twinned, distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive.
Well-formed crystals typically form as a druse, of which quartz geodes are particularly fine examples. The crystals are attached at one end to the enclosing rock, and only one termination pyramid is present. However, doubly terminated crystals do occur where they develop freely without attachment, for instance, within gypsum.

Varieties

Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent or translucent. Colored varieties of quartz are common and include citrine, rose quartz, amethyst, smoky quartz, milky quartz, and others. These color differentiations arise from the presence of impurities which change the molecular orbitals, causing some electronic transitions to take place in the visible spectrum, emitting colored light.
Quartz varieties were previously classified into three categories based on the visibility of their individual crystals. Macrocrystalline quartz varieties have individual crystals that are visible to the unaided eye. Microcrystalline quartz varieties are aggregates of tiny crystals that can only be seen through a microscope. Cryptocrystalline quartz varieties are aggregates of crystals that are too small to be seen even with an optical microscope. Today, the microcrystalline and cryptocrystalline varieties are commonly grouped together and referred to as chalcedony. However, in the scientific literature, chalcedony is a specific form of silica consisting of fine intergrowths of both quartz and its monoclinic polymorph, moganite. Chalcedony is commonly translucent to opaque, while the macrocrystalline varieties of quartz tend to be more transparent. Color is a secondary identifier for the cryptocrystalline varieties and a primary identifier for the macrocrystalline varieties.

Name	Color	Cause	Description	Crystal visibility	Transparency	Major sources	Photo	References
Agate	Frequently multicolored; commonly colorless, pale blue to black, red to orange, yellow, white, brown, pink, purple; rarely green	Varies by color	Banded variety of chalcedony	Cryptocrystalline, microcrystalline	Translucent to opaque	Widespread	Agate nodule from Malawi
Amethyst	Purple to violet	Natural irradiation and trace impurities of iron	Commonly occurs in large clusters and geodes	Macrocrystalline	Transparent	Brazil, Mexico, Uruguay, Russia, France, Namibia, Morocco	Amethyst cluster from Siberia
Ametrine	Violet and yellow	Iron impurities	Commonly believed to be a combination of citrine and amethyst in the same crystal, although the yellow quartz component may not be true citrine. Most material sold as ametrine is partially heat-treated or artificially irradiated amethyst.	Macrocrystalline	Transparent to translucent	Bolivia, Brazil, India	Rough ametrine from Bolivia Cut ametrine
Carnelian	Orange to red, red-brown	Iron oxide impurities	Variety of chalcedony. Natural carnelian is usually light in color; darker colors are produced by artificial heat treatment.	Cryptocrystalline, microcrystalline	Translucent to opaque	Peru, Sri Lanka	Natural carnelian from New Jersey, U.S. Carnelian cabochons
Chalcedony	Almost any color	Varies by color	Fibrous form of silica composed mostly of quartz with some intergrown moganite, occurs in many sub-varieties	Cryptocrystalline, microcrystalline	Transparent to opaque	Widespread	Chalcedony from Czech Republic
Citrine	Natural: yellow to yellow-green or yellow-orange, often with smoky hues Heat-treated amethyst: yellow-orange, orange, red, brown	Natural: no scientific consensus Heat-treated amethyst: trace amounts of iron oxides	Natural citrine is rare; most material sold as citrine is heat-treated amethyst or sometimes heat-treated smoky quartz. Quartz colored yellow from stains, coatings, or inclusions is generally not considered citrine.	Macrocrystalline	Transparent	Brazil	Twinned natural citrine crystals from Russia "Citrine" geode
Cotterite	Silvery metallic sheen	Develops in very thin layers with extremely thin cracks that produce a light-scattering effect giving cotterite a pearly metallic luster	Extremely rare. Derived from a single vein of calcite, quartz and ferruginous mud in Carboniferous Limestone in Rockforest, County Cork, Ireland	Macrocrystalline	Opaque	Ireland	Cotterite from Ireland
Dumortierite quartz	Blue, shades of purple and gray	Mineral inclusions	Contains silky inclusions of blue dumortierite	Macrocrystalline	Translucent		Dumortierite quartz from Brazil
Jasper	Typically red to brown; may have other colors		Impure variety of chalcedony	Microcrystalline	Opaque		Red jasper from Japan
Milky quartz	White	Minute fluid inclusions of gas, liquid, or both, trapped during crystal formation	Less desirable as a gemstone	Macrocrystalline	Translucent to opaque		Milky quartz from Colorado, USA
Onyx	Black and white, monochromatic	Carbon impurities	Variety of agate	Cryptocrystalline, microcrystalline	Semi-translucent to opaque		Onyx from Germany
Prase	Leek green	Inclusions of the amphibole mineral actinolite	As originally defined in Germany. The name prase has also been used historically for similarly-colored quartzite and jasper, and today it may refer to any leek-green quartz.	Macrocrystalline			Prase from Tuscany, Italy
Prasiolite	Green	Trace Fe²⁺ compounds	Rare. Most material sold as prasiolite is produced by heating amethyst.	Macrocrystalline	Transparent	Brazil; Thunder Bay, Canada; Poland	Cut prasiolite from Brazil
Rock crystal	Colorless	Absence of impurities		Macrocrystalline	Transparent to translucent		Clear quartz crystals
Rose quartz	Pale pink to rose	Microscopic inclusions of a fibrous mineral related to dumortierite Euhedral rose quartz: aluminum and phosphorus color centers	Rose quartz is always massive and anhedral. However, a distinct variety called euhedral rose quartz or pink quartz occurs as well-formed hexagonal crystals.	Macrocrystalline	Translucent Euhedral rose quartz: transparent		Rose quartz Euhedral rose quartz cluster from Minas Gerais, Brazil
Rutilated quartz	Clear with golden-yellow or black inclusions	Mineral inclusions	Contains acicular inclusions of rutile	Macrocrystalline	Transparent to translucent		Rutilated quartz cluster from Brazil
Smoky quartz	Light to dark gray, brown, black	Color centers around aluminum impurities activated by natural irradiation		Macrocrystalline	Translucent to opaque		Smoky quartz from Brazil
Tiger's eye	Gold, red-brown, blue		Exhibits chatoyancy	Macrocrystalline	Opaque		Rough tiger's eye Polished red tiger's eye