Ruby and Sapphire

Rubies and Sapphires are both type of the mineral 'Corundum'. In traditional gemology, the difference is that a 'ruby' is red, and a sapphire is any other color.  However, in laser science, a 'laser ruby' is any corundum with chromium in it (regardless of whether it's red or just pink).  Laser crystals using a different colorant (like titanium) will be called 'Laser sapphire' even if they end up being red.