The Orthanc-stone was one of the seven palantíri brought to Middle-earth by Elendil, Isildur, and Anárion.

It was placed in the tower of Orthanc, and when Saruman took up residence there he found it.

During the War of the Ring, Saruman used the Orthanc-stone to communicate with Sauron in Mordor, together plotting against the free peoples of Middle-earth.

The Orthanc-stone was also that which Gríma threw down from the tower of Orthanc when the victors from the Battle of the Hornburg stood upon his master's doorstep. It was picked up by Pippin who looked into it and was revealed to Sauron.

The Orthanc-stone was passed to Aragorn who also looked into it, showing himself to Sauron, so filling the Dark Lord's mind with doubt and fear that the Heir of Isildur had indeed returned.

After the War of the Ring, the Orthanc-stone was returned to the tower of Orthanc by Aragorn after he took up the kingship of Gondor.
Encyclopedia entry originally written by Nienna-of-the-Valar