Allochromatic gems are gems which have been coloured by impurities. If they were totally pure, they would be colorless. The colour of all allochromatic gems is produced by defects. Hence, all shade in corundum is caused by defects. This allows to attenuate humidity and carbon dioxide characteristics, strengthen sign-to-sound ratio, https://bookmarkport.com/story20163622/sri-lankan-yellow-sapphire-for-dummies