You mention that Rust Monsters (And presumably Monarchs) turn into metal statues.

What metal, exactly? Is it just an amalgamation of all the metal it has eaten, in a partuicular length of time or something else? How much of the Metal it has eaten will "become" the statue?

Can a Rust Creature consume other metals, such as Silver, Gold or Mithral?

If so, and you theorecticaly kept a Rust Monster and fed it such a metal for it's entire life, how much of the metal you feed be recoverable, as either the statue or in... waste products?