Pure topaz is colorless and transparent but is usually tinted by impurities; typical topaz is wine, yellow, pale gray, reddish-orange, or blue brown. It can also be made white, pale green, blue, gold, pink (rare), reddish-yellow or opaque to transparent/translucent. Some imperial topaz stones can fade on exposure to sunlight for an extended period of time. Naturally occurring blue topaz is quite rare. Typically, colorless, gray or pale yellow and blue material is heat treated and irradiated to produce a more desired darker blue. Mystic topaz is colorless topaz which has been artificially coated giving it the desired rainbow effect. It Has a mohls hardness of 8 and a refractive index of 1.609-1.643