Quartz — The Most Versatile Gem Species in the Trade
Quartz — The Most Versatile Gem Species in the Trade
Silicon dioxide in two structural families, host to amethyst, citrine, agate, and the entire chalcedony branch
Quartz is silicon dioxide, SiO2, the second most abundant mineral in the Earth's crust and the host species for a larger range of commercial gem varieties than any other mineral. The species occurs in two structural families: macrocrystalline quartz, which forms visible crystals and includes amethyst, citrine, ametrine, rose, smoky, and rutilated quartz; and cryptocrystalline quartz, which forms aggregates of microscopic fibres or grains and includes chalcedony, agate, jasper, carnelian, chrysoprase, onyx, and bloodstone. Both families share the SiO2 composition, hardness 7 on the Mohs scale, and absence of cleavage that make quartz one of the most wearable and economically important gem species.
Mineralogy and physical properties
Quartz crystallises in the trigonal system, typically as hexagonal prisms terminated by rhombohedral faces. Refractive indices are 1.544 and 1.553, with a uniaxial-positive optic character and birefringence of 0.009. Specific gravity is 2.65 for pure quartz and varies slightly across coloured varieties depending on trace-element content and structural state. The species is brittle, with conchoidal fracture and no true cleavage; this combination of hardness 7 and absent cleavage makes quartz one of the most durable common gem materials and explains its dominance in everyday jewellery.
At about 573 degrees Celsius, ordinary low-temperature alpha quartz inverts to high-temperature beta quartz, with attendant changes in symmetry and optical character. The inversion is reversible on cooling and is rarely encountered in jewellery work, but it is significant for the heat-treatment regimes used to alter the colour of amethyst and other varieties.
Macrocrystalline varieties
Amethyst is the purple variety, coloured by trace iron in irradiation-induced colour centres. Citrine, the yellow-to-orange variety, takes its colour from a related iron mechanism. Most commercial citrine is heat-treated amethyst from Brazil; natural citrine is uncommon. Ametrine combines amethyst and citrine in a single crystal and comes principally from the Anahí mine in Bolivia.
Rose quartz is coloured pink to rose-red by trace titanium in fibrous inclusions; the species is normally massive and translucent rather than transparent, though faceted rose quartz from gem rough does occur. Smoky quartz takes its grey-to-black colour from natural irradiation acting on aluminium impurities. Rock crystal is colourless quartz, used both as a faceted gem material and historically for spheres, intaglios, and decorative carving.
Rutilated quartz contains needle-like inclusions of golden rutile, often forming radiating starbursts; tourmalinated quartz contains black tourmaline needles. Both are collector varieties prized for the visual interest of the inclusions. Star quartz, rare and chiefly Sri Lankan, displays asterism when cut en cabochon, the result of oriented rutile or other needle-like inclusions.
Cryptocrystalline varieties
Chalcedony is the umbrella term for cryptocrystalline quartz; it is technically an intergrowth of quartz and the related silica polymorph moganite. Chalcedony in its sensu stricto trade meaning denotes uniformly translucent grey-blue to white material; agate is the banded variety, jasper the opaque variety with high impurity content. Carnelian is orange-to-red translucent chalcedony, coloured by iron oxides; chrysoprase is apple-green chalcedony, coloured by nickel.
Onyx in the strict mineralogical sense is black-and-white banded agate; the term is also applied loosely to dyed black chalcedony and even, confusingly, to certain calcite formations. Bloodstone is dark-green chalcedony with red iron-oxide spots. Sard is brown chalcedony, and sardonyx is brown-and-white banded sard.
Treatments and disclosure
Heat treatment is routine across the macrocrystalline varieties and is generally undisclosed at the rough and parcel level, since the trade assumes citrine and most pale to medium amethyst have been heated. Irradiation produces or deepens colour in some smoky and amethyst material and must be disclosed when applied. Quench-crackling followed by dyeing is used to produce 'fire and ice' or 'cracked' quartz; this treatment is permanent but must be disclosed under AGTA and CIBJO disclosure standards. Dyeing is the dominant treatment for chalcedony, agate, and jasper varieties, and most coloured chalcedony in the commercial trade is dyed.
In the trade
Quartz varieties span the full price range from inexpensive amethyst commercial calibre to fine ametrine, top-grade rutilated quartz, and exceptional chrysoprase that command meaningful prices per carat. Volume production centres on Brazil for amethyst and citrine, Uruguay for fine deep-purple amethyst, Bolivia for ametrine, Madagascar for rose and rutilated material, and India and Sri Lanka for chalcedony varieties. The species' durability and price accessibility make quartz the most-cut gem material globally by volume; it is the commercial backbone of much of the coloured-stone calibrated-gem trade.
For Skyjems clients selecting quartz, the practical hierarchy is colour saturation first, clarity second, cut and origin where they apply. See also amethyst, citrine, ametrine, rose quartz, smoky quartz, rutilated quartz, and star quartz for the variety-specific entries.