The Palantíri or SeeingStones were crystal globes, made by Fëanor in Aman. They showed scenes far away in space (and eventually back in time), but especially things near to another Palantír, thus two Palantíri could be used for visual communication (between two stones in accord  thought was transferred and perceived as speech).
There existed at least eight stones: The Eldar gave seven stones to Amandil, one more stone is mentioned - the Master-stone - which remained in TolEressëa in the tower of Avallónë.
After the downfall of Númenor, Elendil and the remainder of the Númenoreans brought the Seven Stones to Middle-earth and they were used by them for communication among the chief Dúnedain throughout the ThirdAge.
Elendil's three stones were originally placed in EmynBeraid (in Elostirion, the tallest of the three WhiteTowers at EmynBeraid), AmonSûl (Weathertop) and in the SunsetTower? of Annúminas. Isildur's and Anárion's four stones were placed in MinasIthil, MinasAnor, Osgiliath and in Orthanc.
During the ThirdAge several of the stones were lost or destroyed:
Of the remaining stones the following is known:
- The stone of EmynBeraid - the one that was always looking west - was maintained by the Eldar after Elendil's death and finally (by the Keepers of the ThreeRings) brought back to Valinor on their WhiteShip?.
- The Anor stone - used by Denethor II before or during the War of the Ring - remained in the possession of King Elessar, but was no longer useful, because it would only show two burning hands (unless someone with great strength of will could turn it to other purpose).
- The Orthanc stone fell 2759 - casually, how I think - into the hands of Saruman, when he was given the keys of Orthanc (by that time the Palantír was long disregarded) by Beren the Steward. This stone was then "given to" - or better: thrown at - Gandalf, and finally remained also in the possession of King Elessar.
 Usually only the surveyors of two stones (which were in accord) could communicate with each other, however Foster states in the CompleteGuide:
At the moment I cannot clearly decide what the source for this statement is. Maybe it derives from either one of those statements:
|...only the surveyor using the Master Stone of Osgiliath could "eavesdrop."|
|UT - The Palantíri|
In HoMeVII (The Palantír) we find: