IIRC from my last chemistry class, magnetism is based on spin properties of the electrons in an element. Iron is very magentic because it has the greatest number of electrons in a sympathetic spin. Other metals less so, because their orbitals are arranged differently. Given that we're working with imaginary things that could be explained in many ways, I don't see a reason why a metal like adamantite couldn't be a different element that has electric properties similar to iron.