Fourth Doctor's Time Ring

A Time Lord messenger 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)

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

History
After landing the TARDIS on Space Station Nerva (TV: The Ark in Space) the Fourth Doctor was temporarily separated from his ship when he, Sarah Jane Smith and Harry Sullivan used the station transmat to go down to Earth. (TV: The Sontaran Experiment)

Before the trio could return to Nerva, they were intercepted by the Time Lords and taken to Skaro, tasked with the mission of averting the Daleks' creation. The Time Lord messenger issued the Doctor with the Time Ring which he could use to return to Nerva and the TARDIS once his mission was completed. It was his "lifeline" and he was warned not to lose it, but it was confiscated when the Doctor and Harry were captured by the Kaleds, who did not know of its significance. It was eventually recovered from Nyder. Though the Doctor failed to destroy the Daleks at birth, he estimated he had delayed their development and departed Skaro with the Time Ring. (TV: Genesis of the Daleks)

As time was rewritten following the Doctor's intervention of Skaro, the Time Ring overloaded in flight and its fail-safe came into operation. The Time Lords were able to direct the Doctor, Sarah and Harry to the Adelphine Cluster during a war, but all three were separated from each other. After peace was established in the region, the Doctor recovered the Ring which had been stored away by Director Kambril. The Time Lords attempted to bribe the Doctor with a new TARDIS, more reliable than his old one, but the Doctor refused and instead requested Brastall reactivate the Time Ring. (PROSE: A Device of Death)

The Time Ring briefly transported the trio to the Desolii, "the most haunted ship in the galaxy". They used it to depart once again. (PROSE: The Last Thing You Ever See)

After successfully returning them to Nerva Beacon, the Time Ring vanished, to the astonishment of Harry who had wanted to keep it as a memento. (TV: Revenge of the Cybermen)