Allochromatic gems are gems which have been coloured by impurities. If they ended up absolutely pure, they might be colorless. The color of all allochromatic gems is made by defects. Therefore, all shade in corundum is attributable to defects. Certification: The presence of a credible gemological certification from a reputable https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share