The Palantíri - J.R.R. Tolkien

The Unfinished Tales
The Palantíri
Audiobook

The palantíri were made by the Ñoldor in Eldamar, likely by Fëanor himself during his time in Aman in the Years of the Trees, and then given by the Elves to the Faithful Númenóreans, who kept them as heirlooms until the Fall of Númenor during the late Second Age. Seven of these stones were rescued and brought to Middle-earth by Elendil and his sons and set in well-guarded Towers throughout the Realms in Exile.

The Dúnedain placed the stones across large distances to communicate with one another. The stones were housed at these locations: Annúminas, Amon Sûl, and Elostirion in Arnor, and Osgiliath, Orthanc, Minas Ithil, and Minas Anor in Gondor. There was a Master-stone, separate from the Seven, which resided in Tol Eressëa, in the Tower of Avallónë.

Four of the stones are known to have been lost. The chief stone of the north, at Amon Sûl, and the stone from Annúminas were lost with Arvedui in the Icebay of Forochel in TA 1975. The chief stone of the south, in Osgiliath, was lost in the Kin-strife. The stone of Minas Ithil was captured by Sauron in TA 2002 and very likely destroyed in the destruction of Barad-dûr.
The stone of Elostirion was taken back to the Undying Lands on the Ring-bearers' ship. Only the stones of Minas Anor and Orthanc remained in Middle-earth, yet the stone of Minas Anor was marred, showing all but the most strong-willed the burning hands of Denethor II.
The palantíri were most readily used by the Heirs of Elendil, as well as those appointed to guard them; the stones responded best to those with the right to use them, hence Denethor's ability to utilize the stone of the White City easily, whereas Saruman struggled with its use, and was eventually overcome by the superior will of Sauron