Smoky quartz includes quartz of a light brown or dark brown colour, greyish brown quartz, and the black variety called morion. Brown or smoky quartz from the Cairngorm Mountains of Scotland is called cairngorm. When irradiated, colourless quartz may change colour to greyish brown, suggesting that brown quartz may have been formed by natural radiation within the ground. Brown quartz crystals are hexagonal prisms with pyramidal ends, in which inclusions of the mineral rutile may be present.

Crystals weighing as much as 300kg have been found in Brazil. Other localities include Madagascar, the Swiss Alps, Colorado (USA), Australia and Spain.

Much of the smoky quartz on the market is in fact irradiated rock crystal. Smoky quartz has been confused with andalusite, axinite, idocrase, and brown tourmaline.

Chemical Composition: 

Silica (Silicon Dioxide) SiO₂

Colors / Varieties: 

Brown, Grey

Crystal System / Forms: 

Trigonal System

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: 

Brown / Reddish brown

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: 

Not characteristic.

Cause of Color: 

Color center due to the presence of aluminium.

Treatment (Enhancement): 

• 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. Smoky quartz to yellow
• Irradiation:
  1. Rock crystal to smoky quartz

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:

• Identification: type of twinning, seed plate, breadcrumb inclusions, Raman / infra-red spectroscopy.

Simulants (with separation tests): 

Synthetic quartz (inclusions, infra-red spectroscopy), glass (optic character), zoisite (R.I., S.G.), tourmaline (R.I., S.G., inclusions), natural spinel (optic character)

Geological Occurrence: 

Very widespread, in pegmatite.

Sources: 

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

Cuts & Uses: 

Facetted cuts, cabochon, carvings, beads, etc.