The Round Crystal is an item from Phantom Hourglass found in the Temple of the Ocean King. The item appears in a Treasure Chest on Floor B7 of the Temple. Link will need to carry the crystal to Floor B8, where it fits on a pedestal in the northwest corner, needed to reach the Triangle Crystal. Eventually Link will need to carry all three types of crystals and place them in corresponding pedestals on Floor B9.
On Floor B9 of the Temple of the Ocean King, if Link places the crystals in the proper order, the elevator will activate, allowing Link to reach Floor B10. The correct order is Square, Round, and then Triangle.
Each time Link re-enters the Temple of the Ocean King, the crystals will revert to their original locations. In subsequent visits of the Temple of the Ocean King, Link will not need to place the Round Crystal on Floor B8, as a shortcut with the Bombchus will skip this step.