Adamantium

Adamantium is a synthetic metal that is indestructible.

History
Little is known about the history of adamantium.

Wolverine had adamntium bonded to his entire skeleton by Weapon X making him extremely durable. Lady Deathstrike also had adamantium bonded to her body.

Hammerhead had an adamantium plate in his head that he used for ramming.

Hawkeye kept an adamantium net in an arrow he once used to capture Mandarin and MODOK. Mandarin's rings were apparently unable to affect adamantium.

In the Comics
Adamantium is actually poisonous. Wolverine only survives because of his Healing Factor.

Ultron had a body made of adamantium.

Doctor Octopus's tentacles were made of adamantium.

Nick Fury once claimed he had a cup, part of a jockstrap, made of adamantium though he may have been bluffing or joking.

Magneto used his powers to rip the adamantium from Wolverine's body. Apocalypse rebonded him with more.