Nenya

In J.R.R. Tolkien's fantasy universe of Middle-earth, Nenya, also named the Ring of Adamant and the Ring of Water, is one of the Three[?] Rings of Power of the Elves.

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 Lorien, and not normally visible; while Frodo Baggins can see it by virtue of being a Ringbearer, Sam Gamgee tells Galadriel he only "saw a star through your finger".