Did you know there were I think seven or nine (forget now) original Palantiri in Numenor? They were spread throughout the land in towers and were used to see all of the lands. There was one central controlling "seeing stone".
Did you not know that many of the stones were lost when Westernesse was laid low? Only a few recovered and made it to middle earth.
Did you know that Sauron maintained the controlling stone and that Saruman another? In that way Saruman was bent to Saurons's will.
Just a thought, but Soros (Sarumen) could be in possession of a palantir.
I like to think bigger sometimes. Also like a crazy person.:)
Controlling seeing stone = keystone?