In gemmology, an asterism is an optical phenomenon displayed by some rubies, sapphires, and other gems of an enhanced reflective area in the shape of a "star" on the surface of a cabochon cut from the stone.

Asterism is caused by dense inclusions of tiny, parallel, slender fibers in the mineral which cause the light to reflect a billowy, star-like formation of concentrated light which moves around when the mineral is rotated. This is usually caused by small needles of rutile (titanium oxide) in the case of ruby and sapphire. The stars may exhibit four, six, or more rarely twelve rays. The rays are reflections from the rutile crystal. Rutile can also cause a "cat's eye" effect in gemstones. Some specimens may display much stronger asterism than others, and some specimens may have areas where the inclusions are not present, leaving holes or empty areas in the star.

A distinction can be made between two types of asterism:

• Epiasterism, such as that seen in sapphire and most other gems, is the result of a surface reflection.

• Diasterism, such as that seen in rose quartz, is the result of light transmitted through the stone. In order to see this effect, the stone must be illuminated from behind.