Cat's eye is the popular name for an optical effect known as chatoyancy in the gemological nomenclature. Microscopic needle-like inclusions inside the stone reflect a streak of light that appears to open and close as the stone is rotated -- hence the eye like effect. The effect becomes most apparent under a spotlight or in direct sunlight. Although other minerals including tourmaline, beryl, scapolite, sillimanite and quartz may also display the cat's eye effect, these stones are referred to as tourmaline cat's eyes or quartz cat's eyes etc., and only chrysoberyl is referred to as "cat's eye" with no other designation.
The finest quality cat's-eyes display a sharp silvery white line across the stone that opens and closes as the stone is rotated. "Milk and honey" is a term commonly used to describe the color of the best cat's eyes. The effect refers to the sharp milky ray of white light normally crossing the cabochon as a center line along its length and overlying the honey colored background. The honey color is considered to be top by many gemologists but the lemon yellow colors are also popular and attractive. Cat's eye material is found as a small percentage of the overall chrysoberyl production wherever chrysoberyl is found.
Alexandrite is that variety of Chrysoberyl that can display a color change. If a chrysoberyl cat's eye also displays a color change, it is known as an Alexandrite Cat's Eye.