The term "cat's-eye" is commonly misused in the gem trade. The formation of a "cat's-eye" in any gemstone is a phenomenon due to the presence of fibers. The term is commonly used as an adjective, so we may have cat's-eye tourmaline, cat's-eye apatite, and so on. Throughout history, however, the term "cat's-eye" has been used alone in reference a single gem: Chrysoberyl.

Cat's-eye Chrysoberyl is usually yellowish or greenish in color, and transparent to translucent. The most highly valued is a "honey" color, a slightly brownish-yellow hue. A distinctive feature of cat's-eye Chrysoberyl is the strong "relief" of the eye-that is, the intensity with which it shines forth in the stone. Often the eye has a bluish cast.
Gem cat's-eye Chrysoberyl is almost never mined in situ, hut nearly always recovered from stream deposits. The finest gems come from Sri Lanka and Burma, and good material is found in Minas Gerais, Brazil. The best material is "honey-colored" brownish-yellow, followed in value by green, dark brown, pale yellow, and greenish-yellow. The eye must be well centered and sharp, and the gem must be translucent.

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The other variety of Chrysoberyl of major gem significance is alexandrite. The remarkable feature of this gem is its dramatic change, color: red or violet in incandescent light to green or blue-green sunlight. This change of color is due to the absorption of certain light wavelengths by the material. The finest alexandrite changes from purplish red to blue-green, both colors showing a minimum brownish ¬tint. Such spectacular stones come from the U.S.S.R., and some ma -even appear emerald-green in daylight. Stones from Sri Lanka tend to i, more of an olive-green color.
Alexandrite was named after Alexander II, Czar of Russia, on who birthday the gem was discovered. It is also interesting that red and green were the colors of the old Russian Imperial Guard.