Mithril

Mithril is a fictional metal found in J. R. R. Tolkien's Middle-earth writings. It is described as resembling silver, but being stronger and lighter than steel. It was used to make armour, such as the helmets of the citadel guard of Minas Tirith, and ithildin alloy, used to decorate gateways with writing visible only by starlight or moonlight. Always extremely valuable, by the end of the Third Age it was beyond price, and only a few artefacts made of it remained in use.