Allochromatic gems are gems that are coloured by impurities. If they were being completely pure, they would be colorless. The color of all allochromatic gems is produced by defects. So, all shade in corundum is a result of defects. This aids to attenuate humidity and carbon dioxide features, boost signal-to-sounds https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share