Asterism is a type of chatoyancy in which the viewer sees the intersection of multiple cat’s-eyes, or a “star,” rather than a single cat’s-eye. Tightly packed, parallel inclusions that are oriented in two different directions will cause a star with four rays. If there are three sets of inclusions, the viewer will see a star with six rays.

The most well-known examples of asterism in gemstones are star ruby and star sapphire. The accompanying photo shows a black sapphire with a six-rayed star. Stars can also be found in other gems such as garnet, spinel, diopside, and quartz.