Cat's eye alexandrite. Image: ICA/Bart Curren.
A cat's eye is any of several types of gemstone which, when cut to form a convex surface, shows a thin line of light like a cat's eye (chatoyancy). The commonest is chalcedony; the rarest is cymophane, a green variety of chrysoberyl, found in Sri Lanka. The effect is due to minute parallel inclusions or cavities.