In The Lord of the Rings: The Fellowship of the Ring, the One Ring is made from gold.
Sauron forges the Ring from gold and uses it to contain a major part of his own power. Its plain appearance conceals its importance: when heated, it reveals the inscription written in the Black Speech of Mordor. The Ring’s material does not make it ordinary, because its substance is bound to Sauron’s power and purpose.
Mithril is a different, exceptionally strong metal associated with places such as Moria and objects including Bilbo’s coat of mail. The One Ring also has fiery properties, but it is not made of fire or iron. Its destruction requires the fires of Mount Doom, where it was forged.