Wouldn't compressing the coal so quickly create sufficient heat? Of course, if Supes started with a fist-sized chunk of coal, he'd end up with a much smaller diamond, wouldn't he? So creating a fifteen-pound diamond wouldn't be possible, even if he is Superman. He wouldn't be able to get both hands around a piece of coal that big.

Life isn't a support system for writing. It's the other way around.

- Stephen King, from On Writing