Palantir

A Palantir, also known as a seeing stone, is a spherical crystalline device that enables the user to see events or communicate over vast distances. Seven were known to exist in Middle-Earth, but were shunned by many after one was acquired by Sauron. This was generally good advice; Saruman had a palantir, and was corrupted by Sauron through it, while Denethor, the Steward of Gondor, was driven mad by the Dark Lord when he used his seeing stone.