Corundum
= Corundum
{title2=$\mathrm{Al}_2\mathrm O_3$}
{wiki}
<Corundum> is the crystalline alpha form of aluminium oxide, $\mathrm{Al_2O_3}$. Its strong bonded structure gives high hardness and chemical durability.
<Salts> or small amounts of impurity ions can alter its optical properties without changing the basic host lattice. <Ruby> and <sapphire> are gem varieties of corundum, demonstrating how composition at the impurity level affects color.