Ruby
= Ruby
{wiki}
<Ruby> is red <corundum>, with chromium ions substituting for a small fraction of the aluminium in $\mathrm{Al_2O_3}$. 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.