While coal density can vary greatly depending on the deposit it originates from, here is some number chugging:
From:
http://wiki.answers.com/Q/What_is_the_density_of_Coal http://en.wikipedia.org/wiki/Diamond If you take the highest density listed on that page: 1.506 g/mL and compare it to the density of diamond: 3.5 g/cm^3 (1 mL is roughly equal to 1 cm^3 for practical purposes) then the volume differential is only just under 2:1. So, a fist size piece of that type of coal would yield a raw diamond about half that volume.
Onto the impurities. I would compare it to water freezing. When a body of water freezes the crystalline lattice is so precisely coordinated that the very act of freezing forces out most impurities. Apply that to this process. If you assume that the collapse of the carbon into the diamond lattice starts from the center and move outward then it could be that the formation of the diamond would force impurities out of the lattice. Or the lattice could contain faults where it couldn't collapse, I don't know which is more likely. On the other hand, depending on the impurities in the coal used, you could have something that is a natural variant of diamond. The presense of Nitrogen can cause a yellow or brown diamond while boron can cause the rare blue diamonds. Therefore, maybe a diamond made from compressed coal could be some very interesting shade that would make the super-hero forged gem even more unique! =)
I did, however, thoroughly enjoy reading through that link and enjoyed looking up enough to be able to add my own attempt to rationalize an illogical stereotype in our fandom, ~.^
