Allochromatic gems are gems which have been coloured by impurities. When they were completely pure, they would be colorless. The colour of all allochromatic gems is made by defects. Thus, all shade in corundum is caused by defects. Excel in Teachers: Primary Yellow sapphire gemstone is not any a lot https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share