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 on his left hand ring finger. (TV: Deep Breath onwards) The ring fell to the ground when he regenerated within the TARDIS control room, having been released as his hands shrunk to the smaller pair of his successor. (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)