When aluminum oxide crystallize, the mineral corundum is formed. Rubies and sapphires are various forms of corundum. Ruby is the red variety of the mineral and corundum of any other colour is sapphire.
Colour is the ruby's most important feature: transparency is secondary. Inclusions, therefore, don't affect the gemstone's quality.
Star rubies and sapphires contain intersecting needle-like inclusions of the mineral rutile that cause the appearance of a six-rayed 'star'-shaped pattern when viewed with a single overhead light source.
Cultured pearls
Fossils
.jpg "Sapphires - Flat drops")
