Polished tiger's eye gemstone
|Silica (silicon dioxide, SiO2)|
|Colour||golden to red-brown|
|Mohs scale hardness||5.5 - 6|
|Specific gravity||2.64 – 2.71|
Tiger's eye (also called tiger eye) is a chatoyant gemstone that is usually a metamorphic rock that is a golden to red-brown colour, with a silky lustre. A member of the quartz group, it is a classic example of pseudomorphous replacement by silica of fibrous crocidolite (blue asbestos). An incompletely silicified blue variant is known as hawk's eye.
Cut, treatment and imitation
The gems are usually cut en cabochon in order to best display their chatoyancy. Red stones are brought about through gentle heat treatment. Dark stones have had their colours improved and been artificially lightened using nitric acid treatments.
Honey-colored stones have been used to imitate the much higher valued cat's eye chrysoberyl (cymophane), but the overall effect is unconvincing. Artificial fibre optic glass is a common imitation of tiger's eye, and is produced in a wide range of colours. Tiger's eye comes primarily from South Africa and east Asia.
Tiger iron is an altered rock composed chiefly of tiger's eye, red jasper and black hematite. The undulating, contrasting bands of colour and lustre make for an attractive motif, and it is mainly used for jewellery-making and ornamentation. Tiger iron is a popular ornamental material used in a variety of applications, from beads to knife hilts.
Tiger iron is mined primarily in South Africa and Western Australia. Tiger's eye is composed chiefly of silicon dioxide (SiO2) and is coloured mainly by iron oxide. The specific gravity ranges from 2.64 to 2.71. It is formed by the alteration of crocidolite.
Other forms of tiger's eye
Serpentine deposits in which are occasionally found chatoyant bands of chrysotile fibres have been found in the US states of Arizona and California. These have been cut and sold as "Arizona tiger-eye" and "California tiger's eye" gemstones. The trade name of pietersite is used for a fractured or brecciated chalcedony containing amphibole fibers and promoted as tiger's eye from Namibia and China.
- O'Donoghue, Michael (1997). Synthetic, Imitation, and Treated Gemstones. Boston, Massachusetts: Butterworth-Heinemann. pp. 125–127. ISBN 0-7506-3173-2.
- Listing of SG of gems and gem simulants, Berkeley.edu
- Flagg, Arthur Leonard (1958). Mineralogical Journeys in Arizona. Scottsdale: F.H. Bitner. pp. 92–93.
- USGS (1908–1909). "Cat's Eye or Tiger-Eye". Mineral Resources of the United States / Department of the Interior, United States Geological Survey (Washington, D.C.: US Government Printing Office) 2: 802.
- Pietersite on Mindat.org
- Schumann, Walter (2009). Gemstones of the World (Fourth ed.). New York, New York: Sterling Publishing. p. 140. ISBN 978-1-4027-6829-3.
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