Allochromatic gems are gems that happen to be colored by impurities. Should they have been absolutely pure, they'd be colorless. The colour of all allochromatic gems is produced by defects. Consequently, all colour in corundum is a result of defects. By subtracting different amounts of the h•-Fe3+ absorption spectrum from https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share