Adamantium

Adamantium (アダマンティウム Adamantiumu), abbreviated as A, is a rare metallic element. It is a highly dense, unreactive metal that is known to be harder than diamond. It is often alloyed with iron to create Adamant Iron, which is even harder than Adamantium.

Etymology
Adamantium comes from Greek ἀδάμας adámas, which means "unbreakable," and the suffix -ium, which is used to denote elements.