The Elendilmir:

A white gem on a fillet of mithril. There were two gems by this name, one taken by Saruman from the body of Isildur, and the other made for Valandil, Isildur's son in Rivendell. Historians believe that the Elendilmir was originally made in Númenor, for Silmarien. The first was found in Orthanc in the Fourth Age. Apparently, the Elendilmir was the symbol of the kinship of Arnor.
Other names for it included:

The Star of Elendil
The Star of the North
The Star of the North Kingdom