The Blue heart diamond is a rare Type IIb diamond, and all naturally colored blue diamonds belong to this group. However, the occurrence of these diamonds is much less than 0.1 % of all natural diamonds. Type II diamonds are nitrogen-free or contain undetectable quantities of nitrogen.
If the diamonds are not only nitrogen-free but free of all other chemical impurities, they are known as Type IIa, which constitute about 1-2 % of all naturally occurring diamonds. However, instead of nitrogen, if they contain trace quantities of another impurity boron, the diamonds are known as Type IIb. Boron atoms incorporated in the crystal structure of the diamond, changes it's absorption spectrum imparting the blue color to the diamonds. The diamonds also become semi-conducting, unlike other diamonds which are non-conductors of electricity.