Sapphire
= Sapphire
{wiki}
<Sapphire> is gem-quality <corundum>, crystalline $\mathrm{Al_2O_3}$, conventionally excluding the red variety called <ruby>. Trace impurities and defect centers can produce several colors.
Synthetic sapphire is also valued as a hard, optically useful substrate and window material. The material's structural identity is aluminium oxide; a color name alone does not specify which impurity or electronic transition is responsible.