J.R.R. Tolkien> The Lord of the Rings / The Silmarillion

Elrond wore a mantle of grey and had a star upon his forehead, and a silver harp was in his hand, and upon his finger was a ring of gold with a great blue stone, Vilya, mightiest of the Three.

-J.R.R. Tolkien
The Return of the King

Vilya was the greatest of the three Elven Rings of Power. It was made of a gold band and set with a single gemstone, in this case a great sapphire. Vilya was called the Ring of Air and was worn by Gil-galad for a time until he gave it to Elrond at the founding of Rivendell. Elrond probably used the ring's power to help maintain the valley and keep out intruders.