Chatoyancy or chatoyance, is an optical effect seen in some opaque or translucent gemstones. Chatoyancy is caused by the fibrous structure of a material, or by the fibrous inclusions or cavities within the stone. Precipitates of rutile titanium oxide cause chatoyance. The rutile precipitates are all aligned perpendicularly with respect to cat’s eye effect.

The effect is similar to the sheen off a spool of silk. The luminous streak of reflected light is perpendicular to the fibers. A gemstone cut with the fibers parallel to the base exhibit chatoyancy. Faceted stones are less likely to show the effect well.

The term Cat’s Eye can only refer to a Cat’s Eye Chrysoberyl. Any other stone needs to be identified like Cat’s Eye Aquamarine. Aquamarine Cats-eyes are fairly rare. It is a collector’s gem. The highly lustrous Aquamarine Cats-eye comes in a variety of Blue shades from sky blue to deep Sea – blue.

A rare form of aquamarine is translucent with a cat’s eye effect, known as a cabochon. This effect resembles the slit of a cat’s eye and appears to move over the surface.