Allochromatic gems are gems which are coloured by impurities. Whenever they have been totally pure, they might be colorless. The color of all allochromatic gems is made by defects. Therefore, all shade in corundum is due to defects. By subtracting different amounts of the h•-Fe3+ absorption spectrum from the spectrum https://pukkabookmarks.com/story18167544/sri-lanka-yellow-sapphire-an-overview