Crantinium

Crantinium was, according to Mark Seven, the "hardest metal in the Universe." The Daleks once made a bomb encased in the metal, which they intended to use against the ADF and the colonists of the planet Omegon.

As the bomb ticked down to detonation, Mark Seven remembered another property of crantinium: it was magnetic. He abandoned his efforts to defuse the bomb and instead quickly propelled it into space to attach to the hull of the nearby Dalek Deep Space Battle Cruiser. Once magnetised to the hull, the crantinium-hulled bomb exploded, wiping out the Dalek presence in that planetary system. (DAN: Exterminate! Exterminate! Exterminate!)