Allochromatic gems are gems that are coloured by impurities. If they have been absolutely pure, they'd be colorless. The colour of all allochromatic gems is produced by defects. As a result, all colour in corundum is because of defects. Exactly what is most unusual is this first demand transfer band https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share