Adamantium

Adamantium (variously spelled adamantine, adamantite or adamant): the name comes from Greek adamas, "diamond". And, indeed, this metal is diamond-hard and much more strong and resilient than diamond to boot. It tends to be even stronger than mithril, although it is usually rather heavy compared to mithril's supernatural lightness. If adamantium isn't of the maaagic! level of indestructibility and is given more down-to-earth properties, then it resembles the real-world metals tungsten and rhenium.