The Palantiri, also known as the Seven Seeing Stones of Middle Earth, are probably one of the most curious and unusual magical objects in all of the realms. These obsidian orbs are a quintessential example of how something that is created for good, and has amazing and powerful abilities, can be twisted and corrupted to suit an evil purpose. These once beautiful creations are highly dangerous and greatly feared by the time the War of the Ring starts near the end of the Third Age, hence why Gandalf is so stern with Pippin when he picks one of the orbs up, and so relieved that it doesn’t cause more trauma and mental scarring when he sees Sauron through it.
The Palantiri were originally created by the great men of Numenor, in order to allow them to keep in touch with each other over the vast distances of their realms, to survey and protect the lands from raiders and enemies who might try to siege or harm their people, and to allow them glimpses into both the past and the future, in order to best chart their course and steer their decisions for the greater good. However, all of these abilities of the stones become deadly when placed in the hands of Sauron, who infects the stones, and uses them to help him corrupt both Saruman, who as the White wizard and head of the council, should have known better, and the Steward of Gondor Denethor, whose weak mind is driven to madness and despair by the horrible fate that Sauron shows him in the orb.
Interestingly though, it may have been possible for the orbs to be cleansed of the dark lords influence once he was defeated by the ring of power being thrown into the flames of Mount Doom. As the Palantiri were creatures with purity and honorable intentions, it may have been possible to revert them back to this state once there is no way for Sauron to reach them. This begs the question then, would they have been able to be put back to their original task of protecting the kingdoms and communicating with others in different realms (such as those who had gone to the Undying Lands) or would this have been too dangerous a risk to take?
The wise and weary beings of Middle Earth like Aragorn as the supreme king, and Gandalf who has been involved in every battle of the War of the Ring, probably wouldn’t have been willing to take any chances, and would have wanted to test the waters and probably check some magic and spells with them first, before freely handing them out, just in case. In terms of the Undying Lands though, only one of the Seven Stones was powerful enough to see all the way to the Grey Havens, because it was designed specifically for Elendil to be able to look back west, across the sea to the lands where his heart belonged.
The other six stones had a limited reach and connection, and were known as the ‘lesser stones’ because they were far more limited in their scope and abilities. So the possessor of the greatest Palantiri could, in theory, communicate with those who have crossed the sea after Sauron is destroyed, and the orb is cleansed.
Unfortunately, when Elrond parts across the sea to the west on one of the last ships heading for Valinor, he takes the most powerful seeing stone with him. This is at the request of Cirdan the Shipwright, whose job it is to keep Valinor safe, and provide passage to the lands via the ships. It is unknown why Cirdan wanted the strongest of the Palantiri out of reach of Middle Earth, perhaps in order to prevent corruption and catastrophe from reaching the Undying Lands in the event that evil were to ever rise again on the mainland.
Either way, the stone is severed from its counterparts and is therefore unable to be used to communicate with those back in Middle Earth. In many ways, this is sad, for lots of the characters whose loved ones are far away from them, for example, Arwen who stayed, and Elrond who departed into the West, or Samwise who stayed to be with Rosie and their children even though Frodo and Bilbo had journeys across the sea, it would have made the parting easier if they had still been able to check in with one another from time to time.
All in all though, it was probably the wisest and correct decision to make, because it is difficult to know if the Palantiri could ever truly have been rid of Sauron's malignant will, or if there would always have been a danger and threat lurking in the background, ready to rise up again when the opportunity presented itself. Many, like Sam and Frodo, were eventually reunited in Valinor later anyway, and those who weren't, like Arwen and her father, found joy and life in other ways.