Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. 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).

Chemical Composition: 

Silica (Silicon Dioxide) SiO₂

Colors / Varieties: 

• Colorless: Rock Crystal
• Violet: Amethyst
• Yellow, Golden: Citrine
• Brown, Grey: Smoky Quartz (Morion)
• Pink: Rose Quartz
• Green: Praseolite
• 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
• Milky Quartz
• Quartzite

Crystal System / Forms: 

Trigonal System

• 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.

Hardness: 

7

Specific Gravity: 

2.63 - 2.68

Cleavage / Fracture: 

No distinct cleavage / conchoidal to uneven fracture.

Optic Character: 

Anisotropic, D.R.; Uniaxial positive. May exhibit a bull's eye optic figure (quartz rotates the plane of polarisation parallel to the c-axis).

Lustre: 

Vitreous.

Refractive Index / Birefringence: 

1.544 - 1.553 / 0.009. Range: 1.535 - 1.560

Pleochroism: 

Dichroism - weak to moderate.

Dispersion: 

0.013

Magnification: 

Liquid and two phase, color zoning, negative crystals, zebra-stripe finger prints (structural), crystal inclusions, green fuchsite mica flakes / platelets in aventurine quartz, rutile / tourmaline needles in sagenitic quartz. Brazil law twinning is seen in natural quartz.

U.V. Fluorescence: 

Variable.

Spectrum: 

Green aventurine: bands at 682nm, 649nm

Cause of Color: 

Rose Quartz: Charge transfer reaction (Ti⁴⁺ and Fe²⁺) as a result of irradiation.

Treatment (Enhancement): 

• Bleaching: lightening of tiger's eye.
• Coating: all colors, thin film of gold on colorless will give a blue color (aqua aura).
• Colorless impregnation: with oil or resin to improve clarity.
• Colored impregnation (dyeing): all colors, quartzite
• Heat Treatment:
  1. Deep pink to light pink
  2. Crackling to produce iris quartz
• Irradiation:
  1. Light pink to deep pink

Specific Tests: 

• 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).

Synthesis: 

Hydrothermal process:

• Colors - colorless, pink, blue, green, etc.
• Identification: type of twinning, seed plate, breadcrumb inclusions, Raman / infra-red spectroscopy.

Geological Occurrence: 

Very widespread, in pegmatite.

Sources: 

Brazil, India, South Africa, Namibia, Madagascar, Zambia, Sri Lanka.

Cuts & Uses: 

Facetted cuts, cabochon, carvings, beads, etc.