Allochromatic gems are gems which are coloured by impurities. Whenever they were being entirely pure, they would be colorless. The color of all allochromatic gems is made by defects. Therefore, all colour in corundum is due to defects. Excel in Lecturers: First Yellow sapphire gemstone isn't any fewer than a https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share