Allochromatic gems are gems which can be coloured by impurities. When they ended up entirely pure, they'd be colorless. The colour of all allochromatic gems is produced by defects. So, all coloration in corundum is because of defects. Certification: The existence of a credible gemological certification from a respected and https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share