catseye

Catseye is a term used for materials that exhibit a chatoyancy, a distinctive, narrow band of light that seems to move across the surface as the light or viewing angle changes. The most well-known example is chrysoberyl cat's eye, a gemstone prized for its pale to vivid golden-green color and a sharp, single eye that appears to glide across the surface.

The catseye effect arises from light reflecting off aligned microscopic inclusions or structures within the material, such as rutile needles or fibrous grains. These inclusions create a reflective band that remains visible while the rest of the stone stays comparatively dark. The strength and clarity of the eye determine the value and desirability of the gem.

In jewelry, cat's-eye stones are typically cut en cabochon, a smooth, rounded shape that best showcases the eye. While chrysoberyl cat's eye is the classic example, other minerals and synthetic materials can display chatoyancy as well. Catseye quartz, glass, and various composite beads are used in decorative objects and costume jewelry to imitate the effect.

The term also appears outside mineralogy in popular culture and astronomy. The Cat's Eye Nebula, or NGC 6543, is a bright planetary nebula sometimes associated with the idea of a glowing, eye-like feature, contributing to the broader metaphor of the catseye effect.