Star of Elendil
Star of Elendil was the name given to either of two noble gems, both of which were also known by the Elvish name Elendilmir. The first dated back to the early centuries of Númenor, and belonged to Silmariën, the daughter of King Tar-Elendil. From her, it passed to her descendants the Lords of Andúnië. The last Lord was Elendil, who escaped the Downfall of Númenor and became High King of the Dúnedain in Middle-earth. He wore the Star of Elendil on his brow in place of a crown, as did his son Isildur after him.
Isildur had only been King for two years before he was waylaid by Orcs and lost in the Great River. The original Elendilmir was lost with him, and so another was made for his descendants the Kings of Arnor. This second Star of Elendil became the royal symbol of the North-kingdom, and after that kingdom's fall it remained an heirloom of the Heirs of Isildur. It remained in Isildur's line down to the time of Aragorn, who wore it during the Battle of the Pelennor Fields.
At the time Aragorn became King of Gondor, it was thought that the first Star of Elendil had been lost for more than three thousand years. However, it was later found that the searchers sent by Saruman, though they had failed in their mission to find the Ring, had uncovered other things, and the original Star of Elendil was rediscovered in a secret chamber in Saruman's Tower of Orthanc.