From Tolkien Gateway
Angus McBride - Palantír of Osgiliath

The Stone of Osgiliath, the greatest of the seven palantíri of Middle-earth, was the Stone that stood in Osgiliath, the ancient capital of Gondor. For 1,500 years or more, it was held beneath the Dome of Stars that gave the city its name. While most of the palantíri were small and portable, the Stone of Osgiliath was much larger, and could not be lifted by a Man.

The greatest of the Seeing-stones was also the first to be lost. During the early reign of Eldacar, a rebellion arose that came to be known as the Kin-strife. During this civil war, the city of Osgiliath was besieged and burned by the rebels, and the Stone was lost in the waters of the River Anduin.

The Palantíri
 Stones of Arnor  † Annúminas-stone (Annúminas) · Elostirion-stone (Elostirion - Valinor) · † Amon Sûl-stone (Amon Sûl - Fornost)
Stones of Gondor   † Osgiliath-stone (Osgiliath) · † Ithil-stone (Minas Ithil - Barad-dûr) · Anor-stone (Minas Tirith) · Orthanc-stone (Orthanc - Minas Tirith)
Other  Master-stone (Tower of Avallonë)