Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO
4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO 2. There are many different varieties of quartz, several of which are semi-precious gemstones. Throughout the world, varieties of quartz have been since antiquity the most commonly used minerals in the making of jewelry and hardstone carvings.
The quartz group includes a number of different species / varieties. There are two categories crystalline quartz and cryptocrystalline quartz (chalcedony).
Silica (Silicon Dioxide) SiO
Colors / Varieties:
Colorless: Rock Crystal
Amethyst Yellow, Golden:
Citrine Brown, Grey:
Smoky Quartz (Morion) Pink: Rose Quartz
Rock Crystal with irridescent cracks: Iris Quartz
Red Quartz: Rare Variety
Blue, green (aventurescent): Aventurine Quartz
Quartz Cat's Eye
Star Quartz (Six Ray)
Parti Color Quartz: Ametrine
Sagenitic Quartz (Tourmalinated / Rutilated)
Tiger's Eye / Hawk's Eye
Crystal System / Forms:
Prismatic with pyramidal terminations (rhombohedron, trapezohedron, trigonal, pyramid)
Horizontal striations on the prism face, rhomb shaped on rhombohedral face, 'V' shaped markings in twinned crystals.
Most quartz is commonly twinned.
Types of twinning include Dauphine, Brazil and Japan law twinning.
Exhibits both contact and interpenetrant twinning.
2.63 - 2.68
Cleavage / Fracture:
No distinct cleavage / conchoidal to uneven fracture.
; Uniaxial positive. May exhibit a bull's eye optic figure (quartz rotates the plane of polarisation parallel to the c-axis).
Refractive Index / Birefringence:
1.544 - 1.553 / 0.009. Range: 1.535 - 1.560
Dichroism - weak to moderate.
Liquid and two phase, color zoning, negative crystals, zebra-stripe finger prints (structural), crystal inclusions, green
mica flakes / platelets in aventurine quartz,
needles in sagenitic quartz. Brazil law twinning is seen in natural quartz.
Green aventurine: bands at 682
Cause of Color:
Rose Quartz: Charge transfer reaction (Ti
) as a result of irradiation.
Piezoelectric: develops an electric charge when pressure is applied.
Pyroelectricity: develops an electrical charge when heated.
Diasterism in star variety (star visible in reflected and transmitted light).
Colors - colorless, pink, blue, green, etc.
Identification: type of twinning, seed plate, breadcrumb inclusions, Raman / infra-red spectroscopy.
Very widespread, in pegmatite.
Brazil, India, South Africa, Namibia, Madagascar, Zambia, Sri Lanka.
Cuts & Uses:
Facetted cuts, cabochon, carvings, beads, etc.