Definition of "adamantium"

It was a bullet. It was a small slug of adamantium, the toughest and hardest of all metals, crammed to capacity with the terrific explosive feroxite and would burst instantly on any reasonable heavy impact.

1941 June, Malcolm Jameson, “Devil's Powder”, in Astounding Science-Fiction‎, volume 27, number 4, Street & Smith

It's imperative that these experiments be concluded with haste! The military must know the potential of this new adamantium at once! Even the President is standing by!

1969 July, Roy Thomas, “Betrayal”, in Avengers, volume 1, number 66, Marvel Comics

'We knew the egg was adamantium. Not much else could have been learnt.'

2004, Neal Asher, Gridlinked, Macmillan, page 324