Oh, also, are we missing this part of the spell?Stone Shape wrote:While it's possible to make crude coffers, doors, and so forth with stone shape, fine detail isn't possible.
Without proper faceting (which isn't possible using the spell,) there's no way that the final diamond will be worth 20,000 gp. You'll end up with a rough diamond that still needs to be cut and polished. So I think by RAW this idea really doesn't work.
But once again, if you think it's funny, just do it anyway!
Nothing states that it has to be a cut diamond. Only diamond that be worth the set value.
Things like this always get weird when you start to ask value to whom? A large uncut diamond would be worth a great deal to a gem cutter (who can cut it to their own desire). But that is a level of economics that almost no RPG gets into. Gold is always worth "x" per ounce, never changing by location, time, etc..
That being said, the fact that stone shape only works on a single piece of stone at a time kills the whole idea more.
I do think that I like the Pathfinder answer of using diamond dust worth "x" instead of single gems for spells. A little simpler in the long run.