Adamantium is the hardest metal on Earth.
S.H.I.E.L.D. has developed a way to meld adamantium with vibranium. With vibranium being absorbent to any impact and adamantium being the strongest metal in the world, the resulting alloy will be indestructible. Their only known sample and data was stolen by Black Widow, then by Ghost, then given to Obadiah Stane, which he used in the armor plating of Iron Monger.