The One Ring is made of gold in J.R.R. Tolkien’s legendarium. Sauron forged it in the fires of Mount Doom, using his own power to dominate the other Rings of Power and the wills of their bearers.
The Ring looks like a plain golden band, but its nature is revealed by the inscription written in the Black Speech of Mordor. When heated, the fiery letters appear: “One Ring to rule them all...” The gold therefore serves as an ordinary-looking exterior for an artifact of immense supernatural power.
Mithril is a common mix-up because it is another famous Middle-earth metal, especially associated with Bilbo’s coat and the mines of Moria. Mithril is exceptionally light and strong, but it is not the substance of the One Ring. The Ring’s destruction requires the volcanic fire of Mount Doom where it was made.