Asterism is a well-known optical phenomenon that can be observed in a wide range of cabochon-cut gem materials, but is most commonly associated with corundum (i.e. ruby and sapphire). The effect is viewed from above as a four, six or twelve-rayed star-like pattern, which is essentially multiple chatoyancy formed from two or three independent cat’s eye bands that bisect at the center. The vast majority of asterated gem materials display epiasterism, whereby the phenomenon arises due to the reflection of light from multitudes of needle-like inclusions within the material. However, rose quartz’s star effect is only visible when transmitted light is shone through from behind the stone, and is thus a rare example of diasterism.

Asterated specimens are usually given the varietal name “star” followed by that of the specific gem species and/or variety to which they belong (e.g. star sapphire, star diopside, star spinel, star ruby, etc.).