Allochromatic gems are gems which have been colored by impurities. Should they have been totally pure, they might be colorless. The color of all allochromatic gems is produced by defects. Thus, all shade in corundum is attributable to defects. What on earth is most unconventional is this primary demand transfer https://www.tumblr.com/thegemjournal/759696203354128384/understanding-the-unique-characteristics-of-yellow?source=share