They were "unforgeable" because commissioning a jeweler to make an intricate carving on a ring by hand was very expensive in the first place. Finding one who could precisely replicate an existing carving, down to the mistakes of its original author and any wear and tear that would be visible on the wax seal, was almost impossible and even more expensive.

Of course, I didn't mean to imply it took zero effort to copy one, though I guess that's how it read. You'd have to find a skilled jeweler that's also willing to commit treason, so for sure the barrier wasn't insignificant there.

Indeed. In both this scenario and our modern one, we rely on the difficulty of "key material" reproduction being greater than the difficulty of its creation.