Allochromatic gems are gems that are colored by impurities. Should they were fully pure, they would be colorless. The color of all allochromatic gems is made by defects. Thus, all colour in corundum is attributable to defects. Excel in Teachers: Unique Yellow sapphire gemstone is not any below a boon https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share