Master-stone

From Tolkien Gateway

The Master-stone was the chief of all the palantíri, both in Aman and Middle-earth, which was held in the Tower of Avallónë on the Lonely Isle. It was said that the Seeing-stone in Elostirion in the Tower Hills could be used to look back along the Straight Road and glimpse the Tower.[1]

The Stone of Avallónë is variously named the "'Masterstone'" or Master-stone. There was also a 'Master Stone' of Middle-earth, the palantír of Osgiliath, but this was a lesser Stone than the Masterstone in Avallónë, having power only over other palantíri in Middle-earth.[2]

References

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