RE: RE: Link to paper mentioned in MD&AYahweh-Google tells us...
Up to 99% of all natural diamonds are classified as Type I and contain nitrogen atoms as an impurity, replacing carbon atoms within the lattice structure. Nitrogen impurities in Type I diamonds are evenly dispersed throughout the stone, absorbing some of the blue spectrum, thereby making the diamond appear yellow. There are two subcategories (a and b) within each diamond 'type' that are based on a stone's electrical conductivity.
Diamonds that have formed under extremely high pressure for longer periods have a lower nitrogen content, permitting the passage of blue light and making the stone have a 'colorless' (D) appearance. Type II diamonds do not contain detectable nitrogen, thereby allowing the passage of short-wave ultra-violet (SWUV) light through the stone. Natural blue Type II diamonds containing boron impurities and are good conductors of electricity and are classified as Type IIb diamonds, and diamonds that lack boron impurities are classified as Type IIa. Type IIa diamonds have a near-perfect crystal structure making them highly transparent and colorless, with high thermal conductivity. Type IIa diamonds are very rare and some of the finest historical stones such as the Cullinan and Koh-i-Noor are both Type IIa diamonds