Chemistry:Tiger's eye

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Short description
Chatoyant gemstone that is usually a metamorphic rock with a golden to red-brown colour and a silky lustre
Tiger's eye
A polished reddish brown stone which is bisected by a band containing golden fibers
General
CategoryMineral
Formula
(repeating unit)
Silica (silicon dioxide, SiO2)
Identification
Colourgolden to red-brown
Mohs scale hardness6.5-7
|re|er}}Silky
Specific gravity2.64–2.71

Tiger's eye (also called tiger eye) is a chatoyant gemstone that is usually a metamorphic rock with a golden to red-brown colour and a silky lustre. As members of the quartz group, tiger's eye and the related blue-coloured mineral hawk's eye gain their silky, lustrous appearance from the parallel intergrowth of quartz crystals and altered amphibole fibres that have mostly turned into limonite.[1][2]

Other forms of tiger's eye

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.[3] It is formed by the alteration of crocidolite.

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.[4][5] The trade name 'pietersite' is used for a fractured or brecciated chalcedony containing amphibole fibers and promoted as tiger's eye from Namibia and China.[6]

"Photograph of the surface of a stone which shows horizontal alternating bands of red and black with a band of golden-colored fibers in a band across the center"
Tiger iron
"A photograph of a green stone with a pink fibrous band going across the surface diagonally"
Serpentine tiger's eye from Arizona

Sources

Common sources of tiger's eye include Australia, Burma, India, Namibia, South Africa, the United States,[7] Brazil, Canada, China, Korea and Spain.(citation?)

"Photograph of a chunk of rock containing horizontal bands which contain golden fibers which are positioned vertically within the bands"
Unpolished tiger's eye from South Africa

Cultural associations

In some parts of the world, the stone is believed to ward off the evil eye.[8]

Cut, treatment and imitation

Gems are usually given a cabochon cut to best display their chatoyance. Red stones are developed by gentle heat treatments. Dark stones are artificially lightened to improve colour using a nitric acid treatment.[9]

"Photograph of a polished ovoid stone with bands containing shimmering golden fibers"
Oval shape tiger's eye with iron stripes

Honey-coloured stones have been used to imitate the more valued cat's eye chrysoberyl, cymophane, but the overall effect is often unconvincing. Artificial fibre optic glass is a common imitation of tiger's eye, and is produced in a wide range of colours.

References

  1. "Tiger's Eye". http://www.mindat.org/min-3960.html. 
  2. Heaney, Peter; Fisher, Donald (April 2003). "New interpretation of the origin of tiger's-eye". Geology 31 (4): 323–326. doi:10.1130/0091-7613(2003)031<0323:NIOTOO>2.0.CO;2. Bibcode2003Geo....31..323H. 
  3. Listing of SG of gems and gem simulants , Berkeley.edu
  4. Flagg, Arthur Leonard (1958). Mineralogical Journeys in Arizona. Scottsdale: F.H. Bitner. pp. 92–93. 
  5. 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. https://books.google.com/books?id=afI4AAAAIAAJ. 
  6. Pietersite on Mindat.org
  7. Schumann, Walter (2009). Gemstones of the World (Fourth ed.). New York, New York: Sterling Publishing. p. 140. ISBN 978-1-4027-6829-3. 
  8. The Encyclopedia of Superstitions By Richard Webster, p.257
  9. O'Donoghue, Michael (1997). Synthetic, Imitation, and Treated Gemstones. Boston, Massachusetts: Butterworth-Heinemann. pp. 125–127. ISBN 0-7506-3173-2. 

External links