Tiger Eye is a gorgeous, chatoyant, semi-precious gemstone with alternating banks of golden to red-brown colors and a silky luster. It has a fascinating and constantly shifting light effect in which the gold and brown bands appear to keep reversing as the stone is moved relative to the light source. This effect is caused by the light reflecting off of microscopic fibers of crocidolite (a blue form of asbestos) within the stone.
An optical phenomenon, possessed by certain minerals in reflected light, in which a movable wavy or silky sheen is concentrated in a narrow band of light that changes its position as a mineral is turned.
Chatoyance or cat's eye effect, is an optical reflectance effect seen in
certain gemstones, woods, and carbon fibre. Coined from the French œil de chat, meaning 'cat's eye', chatoyancy arises either from the fibrous structure of a material.
|Polished Tiger's Eye Stones. |
Photo: Adam Ognisty
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 forms when Quartz forms over existing bluish-gray Crocidolite, and eventually entirely replace it. Crocidolite is a type of asbestos mineral, which means its composition is of fine, dense fibers. These fibers form in a parallel yet wavy orientation, and this causes the intriguing chatoyant effect exhibited in Tiger's Eye. During the replacement process, the iron within the Crocidolite dissolves and stains the Quartz, thereby providing the golden yellow to brown color of the Tiger's Eye.
When cutting and polishing Tiger's Eye gemstones, skillful orientation to the fibrous structure must be applied to achieve the best chatoyancy. Ideally the cut should be perfectly parallel to the length of the fibers to achieve fullest chatoyancy. Cat's eye effect in Tiger's Eye do exist, but are uncommon in perfect form due to the wavy nature of the fibers.
|Set of various tiger-eye natural mineral stones and gemstones (tigers eye, bull-eye, hawk-eye). Copyright : Vvoennyy|
Other Forms and Types 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.
Hawk's Eye Bluish-gray variant of Tiger's Eye, lacking the yellow or brown color. Hawk's Eye is formed from an incomplete pseudomorphism of Quartz after 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.