Also known as cats eye, this is an optical effect that can be seen on some stones that are cut en cabochon. This phenomenon is caused by needle-like inclusions in a parallel formation. They then produce a single band of light which reflects across the surface of the stone.
Many gemstones can exhibit chatoyancy, e.g. tourmaline, garnet and emerald, but the term ‘cat’s eye’ is most often used in relation to chrysoberyl, as with its greenish-yellow colour and the optical effect, it closely resembles the eye of a feline.