Nenya, also named the Ring of Adamant and the Ring of Water, is one of the Rings of Power, specifically, one of the Three Rings of the Elves of Middle-earth. The name is derived from the Quenya nén meaning water.
Nenya was made by Celebrimbor and the Gwaith-i-Mírdain of Eregion in the Second Age, along with the other two Elven Rings, Narya and Vilya. Their existence was hidden from Sauron, so they were untouched by his evil.
Nenya is described as being made of mithril and set with a "white stone", presumably a diamond (this is never stated explicitly, although the usage of the word "adamant", an old synonym, is strongly suggestive). The ring is wielded by Galadriel of Lórien, and not normally visible; while Frodo Baggins can see it by virtue of being a Ring-bearer, Sam Gamgee tells Galadriel he only "saw a star through your fingers" (This appears in many editions as "finger" - which sounds more magical, since it suggests that her finger has somehow become transparent - but The Treason of Isengard, ch. 13, note 34, mentions it as an error.)
Nenya's power was preservation, protection, and concealment from evil. Galadriel used these powers to create and sustain Lothlórien, but it also increased in her the longing for the Sea and her desire to return to the Undying Lands. After the destruction of the One Ring and the defeat of Sauron, its power faded along with the other Rings of Power. Galadriel bore Nenya on a ship from the Grey Havens into the West, accompanied by the other two Elven Rings and their bearers. With the ring gone, the magic and beauty of Lórien also faded and it was gradually depopulated, until by the time Arwen came there to die in F.A. 121 it was deserted and in ruin.