Is it possible to make crafting materials, like adamantine, out of a goblin or such using the True Polymorph spell for the full duration of the spell? On a side note, while adamantine is mentioned in the Monster Manual, I didn't see it in the PHB. Is that something else that will be fleshed out in the DM guide?
I don't see why not.
It's not exactly a practical way of going about things, but it would be possible (by 17th level money isn't really an object). The spell says you can transform a creature into an object. Turning a Goblin into a lump of adamantine is totally within that realm of possibility (tbh, why stop there, I'd dream a bit bigger, and maybe a bit more valuable, though we don't have a gold price on adamantine in 5e at this time).
When the measuring stick of this is literally comparing it to wish, converting a goblin into a hunk of metal really isn't a big deal.