Yellow Tiger's Eye

Tiger's eye forms in banded iron formations: fractures in the rock were filled with fibrous crocidolite that was enclosed by quartz, and iron oxides trapped in the quartz give the golden colour. It was long explained as quartz replacing crocidolite (a pseudomorph), but Heaney and Fisher (2003) argued that quartz and fibres grew together by a crack-seal vein-filling process. Important deposits lie near Prieska in South Africa's Northern Cape, hosted in banded iron formation of the Asbestos Hills Subgroup (Transvaal Supergroup) and mined on a small scale; there the yellow type is the most sought after. GIA exhibits a slab over 3 m long from the Marra Mamba Iron Formation of Western Australia, formed from sediments about 2.5 billion years old. IGS also lists China, Namibia, England and the United States.

Look for silky bands that switch between light and dark as the stone is turned, on an opaque golden-brown fibrous body. Hawk's eye is the blue, less altered form, and both colours can occur in one piece. In the Mintek grading used in the Northern Cape, golden or honey-yellow and light-brown colour, long straight fibres and sharp chatoyant lines are the most valued. GIA notes that chatoyant glass and plastic made from fused optical fibres are cheap imitations of cat's-eye stones in any colour. Tiger's eye may be bleached or dyed; the Japanese retailer Pascle states that the lighter "golden tiger's eye" it sells is yellow tiger's eye that has been bleached.

IGS rates the wearability of tiger's eye as very good; it has a hardness of about 7 and no cleavage. If a piece has been dyed, GIA advises keeping dyed gems away from solvents such as acetone or alcohol, which can dissolve dyes, and out of prolonged sunlight, which can fade them.

According to Mindat, the first known account of tiger's eye is by the French traveller François Levaillant, who in 1784 found a pebble on the banks of the Orange River with "a varying splendor like the opal or cat's eye" and "a gold-coloured belt". The German naturalist Martin Hinrich Lichtenstein collected tiger's eye and crocidolite along the Orange River in 1803, and the chemist Martin Heinrich Klaproth described them scientifically in 1811; Lichtenstein's specimens, labelled in Klaproth's handwriting, are kept at the Museum für Naturkunde in Berlin.