If you let someone magic up valuable things (and not just conjure something that looks like them) then PCs with thaumaturgy basically don't need the Resources skill anymore.
Sure they do... for one thing- magic transformations should probably still have to pay for longevity (which can get ritually expensive fast). Diamonds eventually decay back to raw carbon- normally over the course a century or two- but if you rush the formation, degradation will probably be a quicker problem too.
As for needing heat and pressure... magic's about taking short-cuts... and it's also a lot about instinct and visualization. Any given zoologist will recognize a werewolf as being
wrong, because holding a perfect image of a wolf's anatomy in one's head to transform into is impossible... but get close enough, and the magic just sort of fills in the gaps to make sure it works
close enough. I'd say the same goes here- you'd be able to create an authentic enough forgery to fool yourself (and by extension, anyone else who's not more of an expert than you are).
I'd almost suggest adding shifts for "authenticity"- ie, the difficulty for identifying the diamond as a forgery.