Smoky Quartz is a brownish black to greyish color variety of quartz. Cairngorm is the Scottish term for smoky quartz.

Gemological information

Gemological Information for Smoky Quartz
Color	Brown to Black or Smoky Grey
Crystal Structure	Hexagonal (trigonal)
Refractive Index	1.544 to 1.553
Durability	Good
Hardness	7
Similar Stones	Andalusite, Tourmaline, Idocrase
Treatments	Quench Crackle, Irradiation, Heat Treatment and Coating or Backing
Country of Origin	Brazil, Scotland, India, Madagascar, Mexico, Srilanka, South Africa, Russia
Smoky Quartz Care
Ultrasonic Cleaning	Usually Safe
Steam Cleaning	Risky
Warm Soapy Water	Safe
Chemical Attack	Soluble in Hydrofluoric Acid and Ammonium Fluoride
Light Sensitivity	Usually Stable
Heat Sensitivity	May Fracture or Change Color

Sources

Gem Reference Guide, Los Angeles: Gemological Institute of America, 1988. Pp. 187-190.

Newman, Harold. An Illustrated Dictionary of Jewelry, New York: Thames and Hudson, 1981. Pp. 283.

Schumann, Walter. Gemstones of the World, New York: Sterling Publishing, Co., 1977. Pp. 116.