The Palantírs

[Bruce MacCulloch]

Footnote in Lord of the Rings, Appendix A,I(iii):

...The only Stone left in the North was the one in the Tower of Emyn Breaid that looks toward the Gulf of Lune. That was guarded by the Elves, and though we never knew it, it remained there, until Círdan put it aboard Elrond's ship when he left. But we are told that it was unlike the others and not in accord with them; it only looked to the Sea. Elendil set it there so that he could look back with "straight sight" and see Eressëa in the vanished West; but the bent seas below covered Númenor for ever.

The tower of Emyn Beraid was one of "the old towers beyond the western borders of the Shire" (Lord of the Rings, "The Shadow of the Past").
Also in "Of the Rings of Power" (Silmarillion)

... Those of Elendil were set in towers upon Emyn Beraid, and upon Amon Sûl, and in the city of Annúminas. But those of his sons were at Minas Ithil and Minas Anor, and at Orthanc and in Osgiliath. ...
It is said that the towers of Emyn Beraid were not built indeed by the Exiles of Númenor, but were raised by Gil-galad for Elendil, his friend; and the Seeing Stone of Emyn Beraid was set in Elostirion, the tallest of the towers. Thither Elendil would repair, and thence he would gaze out over the sundering seas, when the yearning of exile was upon him; and it is believed that thus he would at whiles see far away even the Tower of Avallónë upon Eressëa, where the Masterstone abode, and yet abides. ...