An optical phenomenon whereby rays of light appear in a star shape (usually six-pointed) within a cabochon stone. The best-known asterism is in a star sapphire, though they can also appear in rubies, as well as some other precious stones, including topaz and garnet. There are two types of asterism – epiasterism, where light is reflected by parallel inclusions within a stone, and diasterism, created by light shone through a stone.