The Stone of Orthanc was one of the seven palantíri brought to Middle-earth by Elendil and his sons at the end of the Second Age. The Exiles of Númenor built the Tower of Orthanc on what were then the northern borders of the realm of Gondor, and set the palantír in the unbreachable fortress. As the Third Age wore on, the line of Kings came to an end, and the Rohirrim came to dwell in the lands south of Orthanc, so that in the last years of the Age, the remote Stone of Orthanc was all but forgotten by the Stewards of Gondor. The Wizard Saruman had not forgotten the Stone, though, and when he took over the stewardship of Orthanc a great part of his reason was the hope that the Stone would still be held there.
Saruman was not to be disappointed. When he unlocked the impregnable Tower he found the Stone waiting inside, as it had for thousands of years. In using it, it seems that Saruman had not taken account of the other palantíri in Middle-earth, for the lost Ithil-stone had fallen into the hands of Sauron, and Sauron used its power to bend the Stone of Orthanc to his own will, and with it Saruman himself.
After Saruman's great reverses in the War of the Ring, he lost the Stone in a most peculiar way. While he parleyed with Gandalf from the Tower, his servant Gríma Wormtongue—not realizing the nature of the Stone—cast it at their enemies below, unwittingly granting them a great gift. The Stone of Orthanc was used twice after it was thrown from the Tower, and each time the user encountered the Dark Lord himself. Pippin's use of the Stone was a foolish mistake. The second time, though, Aragorn knowingly used the palantír to reveal his ancestry to Sauron, and his sword Andúril, reforged from the sword that had cut the Ring from Sauron's finger some three thousand years before. Thus he drew Sauron into open war, and distracted Sauron's Eye away from his own land of Mordor, so giving Frodo and Sam a chance to reach Mount Doom.