Rubies and sapphires having a stellate opalescence when viewed down the long axis are termed star ruby or star sapphire.
Emery is a black granular corundum intimately mixed with magnetite or hematite.
Corundum is common as an accessory mineral in some metamorphic rocks, such as crystalline limestone, mica-schist, and gneiss. The finest rubies come from Burma. The best sapphires are found in the alluvial deposits of Thailand, Sri Lanka and Cambodia. Corundum is used as a gemstone and abrasive. The deep red ruby is one of the most valuable of gems.