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: Tiger's Eye

Tiger's-Eye & Hawk's-Eye used in Jewelry

Rough & Polished Tiger's Eye

Source: Australia, Burma, Brazil, Canada, China, India, Namibia, South Africa, USA

Tiger's-eye (aka tigers eye, tiger eye, tigereye, crocidolite, or cat's-eye) is a chatoyant stone made primarily of silicon dioxide (silica), and banded with parallel lines of golden-yellow, and dark yellowish-brown or reddish-brown. It was originally thought that tiger's-eye was a fibrous silicified asbestiform crocidolite that had been substituted with quartz through pseudomorphous replacement.

Tiger's eye consists of quartz (silicon dioxide) which is colored by iron oxide, and crocidolite. Crocidolite (riebeckite, blue asbestos) is a sodium-rich member of the amphibole group of minerals. The tiger's-eye forms from the cracking of the crocidolite host rock, followed by the antitaxial deposition of columnar quartz crystals from the surrounding silica-saturated fluids [2].

Tiger's Eye Jewelry

   Tiger's Eye

Tiger's Eye from India

Tiger's eye derives its name from its color and reflective qualities which are strikingly similar to an actual tiger's eye (iris). There is a blue varieties of silicificed crocidolite that is referred to as "hawk's-eye," which may be a precursor for tiger's-eye (see paragraph below). Hawk's-eye is banded with parallel lines of greyish-blue, and dark indigo-blue. There is also striking reddish-brown, green and blue banded variety that is called "Marra Mamba" (Marra Mumba) that comes from the Hamersley Ranges (Hammersley Ranges) of the Pilbara region, in the northern part of Western Australia.

Material mined in Northern Cape Province, South Africa seems to demonstrate that crocidolite asbestos first transforms into hawk's-eye through silicification, and then into tiger's-eye by partial oxidation of the Fe2+ contained in the crocidolite [2].

According to a 2003 article by Donald M Fisher and Peter J. Heaney, published on the Geological Society of America website [3], the quartz crystal growth in tiger's-eye is synchronous with the crocidolite through a 'crack-seal vein-filling' process: "Our study has revealed that the textures responsible for the shimmer of tiger's-eye do not represent pseudomorphic substitution of quartz after preexisting crocidolite asbestos. Rather, we argue that tiger's-eye classically exemplifies synchronous mineral growth through a crack-seal vein-filling process."

Tiger's Eye Chemistry & Physical Properties

Crystal System hexagonal (quartz), monoclinic (crocidolite)
Crystal Habit pseudomorphous replacement (*)
Specific gravity (SG) 2.64 to 2.71
Mohs Hardness Scale 7
Toughness good
Fracture fibrous
Cleavage none
Streak white
Chemical Composition Silicon Dioxide: SiO2
Chemical Composition Crocidolite: Na2Fe2+3Fe3+2Si8O22(OH)2

Tiger's Eye Optical Properties

Optical Properties uniaxial +, chatoyant
Refractive Index 1.544
Birefringence 0,009
Pleochroism none
Surface Luster silky
Diaphaneity translucent, sub-translucent, opaque
Gem Color brown, greyish-blue, red, yellowish-brown, white

Tiger's Eye is usually cut and shaped into a cabochon to maximize its chatoyant quality. Tiger iron is an altered rock composed chiefly of tiger's eye, red jasper and black hematite. Red varieties of tiger's-eye stones are unusual, and are often created using heat-treatment to enhance color.

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Bibliography and Reference on Tiger's Eye

1. Topgems, Tiger-Eye, Pietersite, Binghamite & Silkstone . www.topgems.homestead.com

2. Jens Gutzmer, The Origin of Tiger's-Eye . www.allenpress.com

3. Donald M Fisher, Peter J. Heaney, Tiger's-Eye . www.gsajournals.org

4. Allison Fisher, Physical Geology - Tiger's-Eye . www.skywalker.cochise.edu

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