Allochromatic gems are gems which can be colored by impurities. Should they have been entirely pure, they might be colorless. The color of all allochromatic gems is made by defects. As a result, all shade in corundum is a result of defects. Wonderful bit of jewellery- properly designed. Not way https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share