Signet ring

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

The Doctor wore the ring on the middle finger of his right hand in his first incarnation, (TV: An Unearthly Child) but soon discarded it after his regeneration into his second incarnation, as it did not fit him any longer. (TV: The Power of the Daleks)

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)

The First Doctor was forced to sell his ring in exchange for French Revolution-style clothes when he arrived in Paris on 25 July, but had it returned to him the next day. (TV: The Reign of Terror)

After the Doctor's first regeneration, the ring no longer seemed to have any use. (TV: The Power of the Daleks)

It was discovered to have fallen into the TARDIS console following the Doctor's first regeneration, where it remained until it was recovered by his seventh incarnation, who decided to start wearing it again. (COMIC: The Chameleon Factor)

The Doctor later gave the ring to Joan Redfern after he turned back from being John Smith. (PROSE: Human Nature)

It was the Other's ring. (PROSE: Cold Fusion)

Behind the scenes
Following the Seventh Doctor's recovery of the ring in The Chameleon Factor, he was then seen wearing it in several subsequent Doctor Who Magazine comic stories — such as The Good Soldier, for example.