Frodo’s protective coat in J.R.R. Tolkien’s The Lord of the Rings is made of mithril.
Mithril is a rare, exceptionally light, and exceptionally strong metal found in Middle-earth. The coat Bilbo gives to Frodo was made by dwarves and consists of small mithril rings. It protects Frodo during the journey and later saves his life when he is attacked in the Chamber of Mazarbul in Moria.
The coat originally belonged to Bilbo, who received it during the events of The Hobbit. Bilbo later gives it to Frodo before Frodo leaves the Shire. Its value is enormous: during the story, Gandalf explains that the mithril coat is worth more than the entire wealth of the Shire.
Mithril is not the same as an ordinary steel shirt or a magical robe. Tolkien presents it as a material with remarkable physical properties, while its protective value comes from its construction and craftsmanship. The name is Sindarin and is commonly translated as “grey glitter.”
The metal also appears in the history of Khazad-dûm, where its mining brought great wealth before the awakening of the Balrog.