Corundum | GRHardnessTester.comCorundum is a mineral that is recognized for its extreme hardness. On Mohs hardness scale, Corundum has a hardness of nine. It is the third hardest mineral, with diamond and moissanite at first and second.  It’s hardness makes this mineral a great abrasive and a great material for cutting gemstones when in the crystal form. Other, more popular names for the mineral are “ruby” and “sapphire.”

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