The fictional metal that makes Bilbo’s mail shirt exceptionally light in J.R.R. Tolkien’s The Hobbit is mithril.
Bilbo receives the mail shirt after Thorin’s company discovers the treasure of the Dwarves beneath the Lonely Mountain. The shirt is made of small mithril rings and is described as light enough for a child to wear but stronger than ordinary steel. Bilbo’s possession of it becomes important when he faces danger during and after the quest.
Mithril is a rare silver-colored metal found in Middle-earth. Its name comes from Sindarin and is commonly translated as “grey glitter.” The greatest known source is Khazad-dûm, later called Moria, where the Dwarves mined it. The metal’s value is so high that it becomes one reason the ruined city remains coveted.
Frodo later wears the same mithril shirt. In The Lord of the Rings, it protects him from a spear thrust in Moria and from other serious blows. Mithril is not the same as the fictional metals galvorn or Valinorean steel.