Corundum is a crystalline form of aluminium oxide.
Present different colors when impurities are present. Transparent specimens are used as gems, called ruby if red and padparadscha if pink-orange. All other colors are called sapphire, e.g., "green sapphire" for a green specimen.

Measures are from http://www.octonus.com/oct/projects/adsorbtion_spectra.phtml#Corundum