There is a certain fascination about having your gemstone not only looking back at you, but giving you the occasional wink. For that reason, the cat's-eye is a favorite among gemologists and gem merchants.
The cat's-eye effect is a lighting phenomenon that gives certain gemstones the appearance of a cat's narrowing pupil—hence the name. It's a bright, thin line of light that stretches from one end to the other of a cabochon-cut gemstone. The bright line is created by the reflection of light off long, thin inclusions. Inside the gem, the reflected light is projected upwards and across the dome of the cab. The phenomenon is also known as chatoyancy. ("Chat" is French for "cat.")