In J.R.R. Tolkien’s legendarium, the One Ring is made from gold.
Sauron forged the One Ring in the fires of Mount Doom during the Second Age. Its gold was not ordinary in narrative importance: Tolkien’s account connects the Ring’s substance with the gold Sauron had used to gain influence over the Dwarves’ Rings of Power. The One Ring contained much of Sauron’s own power and was designed to control the other Rings.
The Ring is often described through its inscription rather than its material. Its black surface bears fiery letters in the Black Speech of Mordor, visible when heated. The familiar inscription begins, “One Ring to rule them all,” in the English translation.
Mithril is a common mix-up because it is a famous precious metal in Middle-earth, especially in connection with the chain-shirt given to Bilbo and later Frodo. Mithril is not the material of the One Ring. The Ring’s destruction required it to be returned to the volcanic fires of Mount Doom.