Necrodermis

Necrodermis is the mysterious "living metal" (even though it comes from Greek roots meaning "dead skin") from which Necrons and their constructs are made. It is more resistant to damage than most materials known to the Imperium and is capable of repairing itself to a certain extent -- melta weapons and other "overkill" weapons will put them down for good, but many Imperial Guardsmen have filled Necron Warriors with lasgun holes and knocked them down, only to see them rise up whole.