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Corundum is the crystalline form of aluminium oxide and one of the rock-forming minerals. Corundum is naturally clear, but can have different colors when impurities are added. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire. The word corundum comes from the Hindi kurand. The oxygen atoms in corundum are arranged in a hexagonal...
- The navel or umbilicus.
- In Greek archaeology: A central boss, as on a shield, a bowl, etc.