Allochromatic gems are gems which are colored by impurities. If they have been wholly pure, they would be colorless. The colour of all allochromatic gems is made by defects. So, all coloration in corundum is caused by defects. Exactly what are you watching for? Go ahead and a dash of https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share