Time Ring
Time Rings were small time travel devices created by the Time Lords. The first Time Rings were worn on the wrist. Later ones were worn on the finger.
The Master duplicated a stolen Time Ring prototype to send his agents to assassinate key figures in Earth's history, but was thwarted by James Stevens, who used one of the Time Rings to prevent Lee Harvey Oswald's death and later to assassinate John F. Kennedy. (PROSE: Who Killed Kennedy)
The Time Lords gave the Second Doctor a Time Ring to see that all of the victims of the War Lords were restored to their proper places in time and space. (PROSE: Players, World Game)
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) When the Doctor and his companions finally returned to Nerva Beacon, the Time Ring disappeared — presumably having been reclaimed by the Time Lords — much to the astonishment of Harry Sullivan, who had wanted to have it as a memento. (TV: Revenge of the Cybermen)
The Seventh Doctor was given time rings by Muldwych to give to Bernice Summerfield and Jason Kane for their wedding (PROSE: Happy Endings) until Dokien retrieved these wedding rings from Bernice and Jason, as momentos. (PROSE: The Collection) However, somehow they still had their wedding rings after this point. (AUDIO: The Grel Escape)
During the invasion of Gallifrey by the Knights of Velyshaa, all Time Capsules and Time Rings were rendered useless. (AUDIO: The Sirens of Time)
The Eighth Doctor was given a time ring by Straxus to rescue Lucie Miller from Hulbert. (AUDIO: Human Resources)
The Tenth Doctor was forced to give an old Time Ring he found in his TARDIS to a Dalek in exchange for it sparing Earth. Kate Yates set it to self-destruct, destroying the Dalek. (PROSE: I am a Dalek)
A Time Ring was used by Harmonious 14 Zink to travel through time "righting wrongs" wherever he and his wife went. This Time Ring was used during and after the events on Erratoon by Zink and Zara to aid in her search for the Key to Time. (AUDIO: The Prisoner's Dilemma) After taking the segment of the Key on Mars, Zara took Zink's Time Ring from him and used it to travel to Safeplace and to Isskar's ship on her own to continue her quest. (AUDIO: The Judgement of Isskar) She used it again when she was a passenger on the Eschaton and during the conflict between the White Guardian and the Black Guardian on Chaos. (AUDIO: The Chaos Pool)