Cat's eye (and tiger's eye) gemstones are a form of quartz, one of mankind's most ancient gemstones. Rock crystal, the clear form of quartz, was used in ancient times to make crystal balls and bowls. Since ancient times colorless quartz crystals have always been popular in jewelry due to mystical legends concerning the "power" of quartz crystals. Quartz crystals exhibit a property called the piezoelectric effect; they produce an electric voltage. This was noticed by ancient cultures, and consequentially since ancient times many mystical properties have been attributed to quartz crystals of various varieties. Cat's-eye chrysoberyl, a close cousin of color-changing alexandrite, is a variety of chrysoberyl which has a distinct band of light across its face which sweeps from side to side. Though the largest deposits of this gemstone are in South Africa, with smaller deposits in Australia, Burma, and India; the most valued deposits, almost exhausted, are those from the Ural Mountains in Russia.