Alumina
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Alumina is the chemical . It occurs naturally as hexagonal crystals called corundum, the second hardest mineral on Earth.[1]
The hardness of corundum by definition 9 on Mohs' scale and it has a white streak, vitreous lustre and specific gravity between 4.0 and 4.1[1]. It is used as an abrasive and the production of refractory.
