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

But Galadriel sat upon a white palfrey and was robed all in glimmering white... On her finger was Nenya, the ring wrought of mithril, that bore a single white stone flickering like a frosty star.

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

Nenya was one of the three Elven Rings of Power forged by Celebrimbor. It was made of a mithril band and set with adamant. Nenya was called the Ring of Water and was worn by Galadriel, who used it to help maintain the land of Lórien.