Asterism in gemstones

Asterism is an optical effect in which light forms a star in a gemstone. It occurs in materials such as star sapphire and star ruby, often cut as cabochons. In corundum, fine oriented rutile inclusions can produce the intersecting light bands. This differs from a cat's-eye effect, which shows one dominant band of light.

For star corundum, GIA identifies a distinct star and contrast against the stone's colour as important qualities. The number of rays varies with material and specimen. A visible star does not prove natural origin: GIA has also examined laboratory-grown star rubies and sapphires.

Inspect the stone while changing the light or viewing angle. Record colour, the star's position and visible damage, ideally with photographs and a short video. Keep the description of the optical effect separate from identification of the material. Natural origin, treatment status and value each require their own supporting evidence.

Sources: GIA: Guide to Phenomenal Gems; GIA: Sapphire Quality Factors; GIA: Large Laboratory-Grown Star Ruby and Sapphire.