Time Ring

Time Rings are small time travel devices created by the Time Lords. The first Time Rings were worn on one's wrist, and later ones were worn on one's finger

The Master duplicated a stolen Time Ring prototype in a plan 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. (MA: Who Killed Kennedy)

The Time Lords gave the Doctor a Time Ring to see if all of the victims of the War Lords were restored to their proper places in time and space. (PDA: Players and World Game)

The Time Lords gave the Doctor a Time Ring to be used to return to Nerva Beacon after carrying out a mission to prevent the creation of the Daleks. (DW: 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. (MA: A Device of Death) When the Doctor and his companions finally returned to Nerva Beacon, the Time Ring disappeared. (DW: Revenge of the Cybermen)

The Doctor gave time rings to Bernice Summerfield and Jason Kane for their wedding. (NA: Happy Endings)

The 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. (TDA: I Am a Dalek)