Cat’s eye topaz
Topaz is a very common gemstone but cat’s eye topaz is rare. The two main sources are Burma (Myanmar) and Madagascar
Pure topaz is colorless and transparent but is usually tinted by impurities, typical topaz is wine red, yellow, pale gray, reddish-orange, or blue brown. It can also be white, pale green, blue, gold, pink (rare), reddish-yellow or opaque to transparent/translucent.
Orange topaz, is the traditional November birthstone, the symbol of friendship, and the state gemstone of the US state of Utah.
Imperial topaz is yellow, pink (rare, if natural) or pink-orange. Brazilian Imperial Topaz can often have a bright yellow to deep golden brown hue, sometimes even violet. Many brown or pale topazes are treated to make them bright yellow, gold, pink or violet colored. Some imperial topaz stones can fade on exposure to sunlight for an extended period of time.
Blue topaz is the state gemstone of the US state of Texas. Naturally occurring blue is quite rare. Typically, colorless, gray or pale yellow and blue material is heat treated and irradiated to produce a more desired darker blue.
Topaz is commonly associated with silicic igneous rocks of the granite and rhyolite type. It typically crystallizes in granitic pegmatites or in vapor cavities in rhyolite lava flows including those at Topaz Mountain in western Utah and Chivinar in South America. It can be found with fluorite and cassiterite in various areas including the Ural and Ilmen mountains of Russia, in Afghanistan, Sri Lanka, Czech Republic, Germany, Norway, Pakistan, Italy, Sweden, Japan, Brazil, Mexico, Flinders Island, Australia, Nigeria and the United States.
Cat’s eye effect
In gemology, chatoyancy, also chatoyance or cat’s eye effect, is an optical reflectance effect visible in certain gemstones. Coined from the French “oeil de chat”, meaning “cat’s eye”, chatoyancy arises either from the fibrous structure of a material, as in cat’s eye tourmaline, cat’s eye topaz, or from fibrous inclusions or cavities within the stone, as in cat’s eye chrysoberyl. The precipitates that cause chatoyance are the needles. Examined samples have yielded no evidence of tubes or fibers.The needles precipitates all align perpendicularly with respect to cat’s eye effect. The lattice parameter of the needles matches only one of the three orthorhombic crystal axes of the chrysoberyl, as a result of an alignment along that direction.
The phenomenon resembles the brilliance of a silk spool. The luminous streak of reflected light is always perpendicular to the direction of the fibres. For a gemstone to show this effect better, the shape must be a cabochon. Round with a flat base, rather than faceted, with the fibers or fibrous structures parallel to the base of the finished gem. The best finished specimens show a single sharply. A band of light that moves across the stone when it turns. Chatoyant stones of lesser quality display a banded effect as is typical with cat’s-eye varieties of quartz. Faceted stones do not show the effect well.
Cat’s eye topaz from Burma