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, in
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.