Animorphs 'solved' this problem with time limits and basically mental conditioning.

Your brain transforms too, but somehow 'you' mostly remain intact.

But... if you're weak minded, the beast takes over, its instincts are too overwhelming, you lose yourself and become the beast.

How this would work with a ... gold bar?

Well the closest thing I can think of would be like a trilobyte, iirc their outer carapace was literally minerals, basically a rock.

But I might be wrong about that.

But if I'm not... gold rectangular trilobyte.