Mithril

Mithril is a precious silvery metal, as strong as steel but much lighter in weight, that was mined by the Dwarves in the mines of Khazad-dûm. The name mithril comes from two words in the fictional Elvish language -- mith, meaning "mist", and ril meaning "glitter".

The Mithril Coat

Warning: Wikipedia contains spoilers

Of all items made of mithril, the most famous is the coat of mail retrieved from the hoard of the dragon Smaug, and given to Bilbo Baggins by Thorin. Bilbo then gifted the coat to his nephew Frodo, who wore it during the Quest to Mount Doom. It saved Frodo's life when he was nearly skewered by a troll in the Mines of Moria. It was later taken by the orcs who captured him in the pass above Cirith Ungol, and passed on to the Dark Lord's servants at Barad-Dur. When the coat was displayed before the hosts of Aragorn at the Gates of Mordor, many despaired, thinking Frodo had been captured or killed, and the Ring taken. Gandalf reclaimed it from Sauron's lieutenant, and was later able to return it to Frodo after the battles were won.

Mithril, or similarly spelled names, has been used in other fictional contexts, as a strong and semi-magical metal; examples include:

• the role playing game Dungeons & Dragons, where mithral is one of the special materials used to make high-quality or magical items,
• the video game Hexen II, in which there is a mithril wall somewhere underwater that must be transformed into wood via a spell in order to proceed in the game,
• the video game series Final Fantasy, as a material for shields, weapons, armour, helmets and so on
• the video game ADOM, as a type of metal, one of the strongest