Sapphire

Sapphire is gem-quality corundum, crystalline $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, 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.