The Doctor's signet ring

The Doctor once owned a signet ring in which was mounted a large blue crystal.

History
The Doctor's signet ring was originally worn by the incarnations of the Doctor who were married to Patience on Gallifrey. (PROSE: Cold Fusion) In an abnormal state of history, the Doctor wore a similar ring. (PROSE: The Infinity Doctors)

The First Doctor wore the ring on the middle finger of his right hand. (TV: An Unearthly Child, Twice Upon a Time) The signet ring was used by the First Doctor in his escape from Gallifrey, which involved the breaching of "laws and barriers". (PROSE: The Three Paths) He was forced to sell the ring in exchange for French Revolution-style clothes when he arrived in Paris on 25 July, but it was returned to him the next day. (TV: The Reign of Terror)

The Doctor used the ring in defeating Jiang at gungfu, (PROSE: The Eleventh Tiger) to store an excess solar energy on an unnamed planet (AUDIO: Starborn) and to open the TARDIS door on Vortis. (TV: The Web Planet)

After regenerating into the Second Doctor, the ring didn't fit his middle finger any more (the digit was now too thick to accommodate it) and he discarded it, (TV: The Power of the Daleks) subsequently becoming more reliant on his sonic screwdriver. (TV: Fury from the Deep) The ring fell into the TARDIS control console.

The Seventh Doctor eventually recovered the ring from inside the TARDIS console, tried it on, and decided to start wearing it again. (COMIC: The Chameleon Factor) After wearing the ring through numerous adventures, (COMIC: The Good Soldier, Metamorphosis, PROSE: Tragedy Day) he gave it to Joan Redfern after he turned back from being John Smith. (PROSE: Human Nature)

The Doctor eventually wore the ring again in his Muldwych incarnation. (PROSE: Birthright)

Abilities
The ring had several special abilities. It could supply power to the TARDIS's main doors and control a Zarbi. (TV: The Web Planet) If given the correct spectrum of light, it could unjam a TARDIS's locking mechanism. (TV: The Daleks' Master Plan) The ring seemed to be able to facilitate hypnotism and protect the Doctor from electrical shocks. (TV: The War Machines) On the Ethereal homeworld, the Doctor used the ring to open an electronic lock. (PROSE: The Cloud Exiles) When the Doctor received lessons in the "magic" of Avalon from Kilvenny Odoyle, the ring served as his "wand" through which he focused his spells. (PROSE: The Sorcerer's Apprentice) On Hogsumm, the Doctor used the ring to temporarily repair the damage bubbleshake had caused to Bernice Summerfield's mind. He later used it as a substitute operator's key for a transmat system. (PROSE: The Highest Science)

Behind the scenes

 * The ring plays an important part in Doctor Who: Legacy's Sonic Adventure storyline. In it, the Master steals all of the sonic devices onboard the TARDIS and scatters them through time. The ring was not stolen because it contained no sonic technology, but the First Doctor could still use it like the other Doctors used their screwdrivers and used it to get back all the sonic devices.