Ring

A ring was a circular item of jewellery which was worn around a finger.

The Doctor wore several rings over his lifetime. The First Doctor wore a signet ring throughout his life, (TV: An Unearthly Child - The Tenth Planet) only for the Second Doctor to discard it upon discovering that it was too large for him. (TV: The Power of the Daleks) The Seventh Doctor later recovered this ring (COMIC: The Chameleon Factor) and began wearing it once again. (COMIC: The Good Soldier, PROSE: Tragedy Day et al)

The Third Doctor adopted a pinky ring which he wore on his left hand, (TV: The Ambassadors of Death onwards) while the Twelfth Doctor wore a ring with an amber gemstone on his left hand ring finger. (TV: Deep Breath onwards) The ring fell to the TARDIS floor when he regenerated within the TARDIS control room, having been released as his successor's hands were smaller. (TV: Twice Upon a Time)

The Master wore numerous different rings throughout his life. Whilst posing as Harold Saxon, the Master wore two rings: one a simple gold band on his left hand ring finger, and another ring, featuring an ornate design resembling the logo of Lazarus Laboratories. (TV: The Sound of Drums/Last of the Time Lords) later wore a variety of rings, which she wore on either middle finger. (TV: Dark Water/Death in Heaven, The Magician's Apprentice/The Witch's Familiar)

Rings could be used as bio-dampers. The Tenth Doctor gave Donna Noble a bio-damper in the form of a ring so that she would be hidden from the Racnoss. (TV: The Runaway Bride) Gavin wore a bio-damper ring he inherited from his father which hid his alien origins. (TV: The Empty Planet)

Eldrad possessed a ring which contained his genetic imprint, which the Fourth Doctor and Sarah Jane Smith used to help him regenerate himself. (TV: The Hand of Fear)

Ibrahim Namin possessed a ring which he used to control Sutekh's Osiran service robots. (TV: Pyramids of Mars)

Clara Oswald wore a variety of rings, on both of her ring and middle fingers. (TV: The Day of the Doctor et al)

John Hart possessed a ring that could emit a tracking signal to last 5,000 years. He threw this ring in with Jack Harkness when he was burying him. (TV: Exit Wounds)