"They are not all accounted for, the lost seeing stones. We do not know who else may be watching." —Gandalf to Saruman, in The Fellowship of the Ring
The Palanlri were seeing stones. There were seven in all and were used to communicate in Middle Earth and beyond.
“The lore of the Stone’s has since been forgotten and can only be partly recovered by conjecture and from things recorded about them. They were perfect spheres, appearing, when at rest, to be made of solid glass or crystal deep black in hue. At their smallest, they were about a foot in diameter, but some, certainly, the Stones of Osgiliath and Anon Sul, were much larger and could not be lifted by one man.”
Unfinished Tales, J.R.R. Tolkien, pg. 472