Allochromatic gems are gems which have been colored by impurities. Whenever they have been totally pure, they might be colorless. The color of all allochromatic gems is made by defects. Hence, all shade in corundum is due to defects. As indicated previously, in addition to the absorption cross area increase https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share