Allochromatic gems are gems which are coloured by impurities. Whenever they ended up completely pure, they might be colorless. The colour of all allochromatic gems is produced by defects. So, all shade in corundum is attributable to defects. A Precious Gift from Mother nature Sapphires are formed numerous decades back https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share