Tiger's eye

Tiger's 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.

Tiger's eye is a microcrystalline, golden-brown to golden-yellow striped variety of the mineral quartz. The translucent to opaque aggregates usually have a clearly visible fibrous structure and exhibit a greasy to silky sheen on their surfaces. Tiger's eye is used almost exclusively as a gemstone, exhibiting a wave-like shimmer, known as chatoyance or the "cat's eye" effect, when cut smoothly. Occasionally, tiger's eye is also used as a guitar pick. 

Tiger's eye

General

Category Minerals

Formula Silica (silicon dioxide, SiO2)

Identification

Colour golden to red-brown

Mohs scale hardness 6.5-7

Luster Silky

Specific gravity 2.64–2.71

Composition

This mineral with yellow and brown tiger tones is formed from Amosite (fibrous variety of Grunerite), it is one of the six varieties of fibers grouped under the commercial name "asbestos". The fibers of this mineral have been partially substituted by silica, by a phenomenon of pseudomorphosis. It belongs to the microcrystalline quartz variety. An opaque and hard stone, with a hardness of 7 on the Mohs scale and a density between 2.64 and 2.71, its shimmering color can range from blue-grey to yellow-brown and red-brown.

Brown stones are generally called bull's-eye and can be obtained by heating a tiger's-eye. There are also similar stones but blue, called falcon's-eye. These blue stones are made of crocidolite, the fibrous form of Riebeckite, which is another of the six varieties of asbestos. They can be altered to yellow tones by artificial heating as well.

Stones of this variety exhibit remarkable chatoyancy, an optical effect formerly known as cat's eye. This effect, combined with the yellow-brown hue, betrays the origin of the name tiger's eye, recalling the animal in question. Tiger's eye and cymophane (chrysoberyl cat's eye) have only the name and the chatoyancy effect in common, as they are two different minerals, although tiger's eye has been used as a (cheaper) imitation of the rarer cymophane: the effect is due to the inherited structure of crocidolite fibers for tiger's eye, while for cymophane it is rutile inclusions.

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 (SiO

2) 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.

Serpentine deposits in the US states of Arizona and California can have chatoyant bands of chrysotile, a form of asbestos, fibres. These have been cut and sold as "Arizona tiger-eye" and "California tiger's eye" gemstones. 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.

Sources

Common sources of tiger's eye include Australia, Burma, India, Namibia, South Africa, the United States, Brazil, Canada, China, Korea and Spain.

Cultural associations

In some parts of the world, the stone is believed to ward off the evil eye. Roman soldiers wore engraved tigers eye to protect them in battle.

Use as a gemstone

Tiger's eye is usually offered in cabochon form in various designs, but also in spherical shapes for necklaces or free-cut for decorative objects. Particularly highly domed cabochons exhibit a wandering streak of light, known as chatoyance, reminiscent of a cat's slit pupil.

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.

Manipulations and imitations

When fired, tiger's eye acquires a strong copper-red color, which can develop into violet when subsequently quenched in spirit. Red tiger's eye is not found in nature and is therefore always fired.

When cooked in strong acid, tiger's eye loses its color and turns gray. In this form, it is occasionally offered as an imitation of chrysoberyl, cat's eye, or cat's eye quartz. 

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. Due to the similarity in color, there is a risk of confusion with tiger iron and the so-called “tiger jasper ”.

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