The optical phenomenon responsible for star sapphire's shimmering rays is called asterism, from the Latin word astrum, for “star.” Asterism is caused by small needle-like inclusions of the mineral rutile. Black star sapphires owe their stars and their dark color to included crystals of the shiny mineral hematite.
These small needles -like inclusions within the sapphire crystal reflect light and form a star sapphires. The most common star pattern is a 6 rayed star, although star sapphires with 4 rays and 12 rays have been found.
The effect is best viewed under a direct light source and while tilting and rotating the stone from different angles.
The most important deposits of fine star sapphire today are found in Australia, Myanmar (Burma), Sri Lanka and Thailand. Other significant star sapphire sources include Brazil, Cambodia, China, Kenya, Madagascar, Malawi, Nigeria, Pakistan, Rwanda, Tanzania, United States (Montana), Vietnam and Zimbabwe.