Supermanium is apparently the hardest metal in the known universe; it is practically as indestructible as the Superman himself.
It was used in the construction of Superman's Supermobile, as well as a jail for Brainiac. According to Superman, he forged it out of the heart of a star, similar to how the Atom obtained his costume. Superman used it to construct a rocket to securely protect radioactive material waste from being exposed on its journey in space.
Links and References