Is there a value limit on the objects True Polymorph creates?

Question

True Polymorph has the option to transform a creature into an object that cannot be larger than the creature. What about the object's value? Wish has a limit of 25,000 gp for on-label use. Could True Polymorph be used to turn a 2 gp sheep into a massive diamond, or a sheep-sized block of platinum?

Answer 1

You can turn it into anything (under the size limit), but you probably won't be able to sell it, and using it is risky

The new highly valuable object is under a permanent effect, not an instantaneous effect. The latter is the only truly permanent change; it's not maintained by magic, so it can't be detected or ended by things that detect or dispel magic.

Since True Polymorph is permanent, it's constantly under the effect of the spell. If you're trying to sell your diamond the size of a sheep, the people you'd be selling it to would almost by definition have the resources for a simple Detect Magic check (if nothing else they can pay someone else to do it for far less than the cost of the diamond), which would reveal an aura of Transmutation magic around it; they'd know it's a fake, and almost certainly refuse to buy it.

Even if you use it yourself to achieve some valuable end (making yourself a suit of adamantine plate mail or whatever), you're just one lucky (or high-level) casting of Dispel Magic away from being suddenly naked with your angry foe restored beside you (or possibly in squishy pieces all over you, depending on how your DM interprets the process of forging a polymorphed foe).

So it works, it's just not going to do you as much good as you might hope.

Probably the best use for it (if your DM doesn't veto) would be making expensive material components during downtime. No need to sell, and depending on the spell, you may be casting it out of combat with no risk of it being dispelled.