Allochromatic gems are gems which are coloured by impurities. When they were absolutely pure, they'd be colorless. The color of all allochromatic gems is made by defects. So, all color in corundum is due to defects. What are you waiting for? Go on and a dash of sunshine to the https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share