Fourth Doctor's Time Ring

The Time Lords gave the Fourth Doctor a Time Ring to return to Nerva Beacon after carrying out a mission to prevent the creation of the Daleks. (TV: Genesis of the Daleks) The temporal distortion caused by the Doctor's actions on Skaro disrupted the Time Ring, scattering the Doctor and his companions across the Adelphine Cluster. After peace was established in the region, the Time Lords gave the Doctor a new Time Ring. (PROSE: A Device of Death)

By one account, the Ring was experimental technology stolen by Deliavatsud from the Temporal Development Center. (PROSE: The Dalek Problem)