Asterism and chatoyancy

A star trapped inside the stone

Asterism is that striking phenomenon in which a four- or six-rayed star appears on the surface of a gem and moves as you tilt the stone or the light. The star sapphire and the star ruby are its most famous examples. Our simulator reproduces the effect: move the light source and watch the star follow.

The secret: microscopic needles

The phenomenon is caused by needle-shaped inclusions, usually of rutile, aligned along the crystal's growth axes. Light strikes these thousands of parallel needles and reflects perpendicular to them, creating one luminous band per needle direction. In corundum (sapphire, ruby), the trigonal crystal system imposes three directions 60° apart: three bands crossing, hence a six-rayed star.

Chatoyancy — the “cat's eye” effect — follows the same principle with a single direction of fibres: light concentrates into one mobile band, exactly like a feline's pupil. Tiger's eye, hawk's eye and cat's-eye chrysoberyl are the classic examples.

Why these stones are always cut en cabochon

This is not an aesthetic choice but an optical necessity: the effect only appears on a domed surface, which gathers the reflections to a point. A faceted stone would scatter the rays and the star would vanish. That is why every star sapphire and every tiger's eye in the world is cut as a smooth dome — and the lapidary must also orient the stone perfectly when cutting, on pain of throwing the star off-centre and ruining the specimen's value.

A field tip: these effects show best under a point source — a torch, direct sunlight — and almost vanish under diffuse light. If a seller shows you a star sapphire under soft lighting, ask for a lamp.