Ruby

Ruby is red corundum, with chromium ions substituting for a small fraction of the aluminium in $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$. The local crystal field modifies chromium's electronic energy levels.

Selective absorption produces the red appearance, and characteristic emission supports the classic ruby laser. This connects a trace substitution in a crystal with a macroscopically visible optical property.