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.
|Cat's eye alexandrite
Image: ICA/Bart Curren
AND PLANETARY SCIENCE