Chatoyancy

Chatoyancy is the optical phenomenon in which a single thin band of light appears to move across the surface of a cabochon-cut gemstone, resembling the slit pupil of a cat's eye. The phenomenon is caused by reflection off parallel fibrous inclusions or hollow channels oriented in a single direction within the host crystal; light from a single source reflects off all the parallel fibers simultaneously and produces a single perpendicular line of light.

The term "cat's eye" without further qualification specifically refers to chrysoberyl cat's eye — the highest-quality and most-traded chatoyant gemstone. Chrysoberyl cat's eye shows a sharp, vivid line that moves across a honey-yellow, greenish-yellow, or apple-green body color as the stone tilts. The finest specimens show the "milk and honey" effect — when illuminated from one side, half the cabochon appears milky-white while the other half retains the honey body color, with the cat's eye line separating the two zones. Top-quality milk-and-honey chrysoberyl cat's eyes trade $3,000 to $15,000 per carat at 3 ct and above.

Other gem species show chatoyancy under specific inclusion conditions: tourmaline (cat's eye tourmaline, lower-priced than chrysoberyl), apatite, aquamarine, scapolite, tiger's eye quartz (the most familiar but lowest-priced chatoyant material). Each species has its own price tier and aesthetic; chrysoberyl is the trade-default "cat's eye" without further qualifier.

The quality of the cat's eye depends on three factors: fiber density (more fibers produce a sharper line), fiber orientation (perfect parallel alignment is required for a sharp single line), and cabochon orientation (the cabochon must be cut with the fiber direction parallel to the base, so the line of light moves perpendicular to the long axis of the dome). Cabochons cut with poor fiber alignment show weak, fuzzy, or broken cat's eye lines.

Chatoyancy is distinct from asterism in that the asterism star is multi-ray (six rays in corundum, twelve in some specimens) while chatoyancy is single-line. The geometric distinction is that asterism requires two or three sets of intersecting parallel inclusions, while chatoyancy requires only one set of parallel inclusions. Some specimens show both phenomena — a chatoyant cabochon with intersecting fibers can show asterism, while a star sapphire with one dominant silk direction can show a strong chatoyant line as well as the star.

The trade values "openness" of the cat's eye — how cleanly the line resolves and how fluid the movement is as the stone tilts. Sharp, open, mobile cat's eyes trade at the top; fuzzy or sluggish lines trade at commercial discounts.