The fabled sapphire, one of the four gems called precious today, is found in shades ranging from royal blue, to pink, green and black. It is an extremely hard variety of corundum (an aluminum oxide) found in transparent or translucent crystals, whose variation in color are caused by the presence of trace minerals. The finest sapphires have been mined since Antiquity in Burma, Thailand, India and other parts of the East. Though the stone was known in the West, it was rarely used by anybody but royalty until the Christian era. Its rarity contributed to its reputation as a magical stone of immense and all encompassing power, capable of warding off every kind of pestilence and contagious disease. Those sapphires, which show asterism—the presence of a rayed star—are the most cherished because it is believed they are capable of conquering al the forces of evil. The sapphire is also reputed to bring good luck to its owner and to promote wisdom and clarity of mind.