The how and why of Tiger’s eye

Deborah Painter (USA) Tiger’s eye is definitely an unusual semiprecious gem because of a phenomenon called “chatoyancy” seen in only a few minerals and stones. “Chat” is of course the French word for “cat”. The golden bands of polished specimens remind one of a cat’s eye. Chatoyancy refers to the way the distinctive bands of yellow and golden brown within the polished stone refract light as one tilts and moves the stone. It seems to possess an inner dimension. That inner dimensionality effect is due to the fibres of crocidolite asbestos (a variety of magnesio-riebeckite) locked within the stone (Fig. 1). Magnesio-riebeckite is composed of silicon, iron and sodium. Quartz is composed of silicon and oxygen. Quartz has impregnated the greater portion of crocidolite within Tiger’s eye and only a small percentage is crocidolite asbestos. This is why Tiger’s eye is known as one of the “pseudomorphs”, which is a mineral that transforms partly into a different mineral. Fig. 1. This specimen of crocidolite from the Mineralogical Museum in Bonn resembles a shimmering holiday decoration. Its beauty is in sharp contrast to its reputation as asbestos, which is listed as most hazardous to health. (Credits: Raimond Spekking.) A hydrothermal metamorphic process created the pseudomorph known as Tiger’s eye. In a hydrothermal condition during metamorphism of the bedrock in which crocidolite occurs, the mineral will experience tiny fractures. Quartz grows on the outer surface of the crocidolite. This process repeats itself until the crocidolite is surrounded completely in quartz and imbedded … Read More