Signet ring

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

The Doctor wore the ring in his first incarnation, (DW: An Unearthly Child) but soon discarded it after his regeneration into his second incarnation, as it no longer fitted him. (DW: The Power of the Daleks)

The ring had several special abilities, such as the ability to supply power to the TARDIS's main doors, and control a Zarbi. (DW: The Web Planet) If given the correct spectrum of light, it could unjam a TARDIS's locking mechanism. (DW: The Daleks' Master Plan) The ring seemed to be able to facilitate hypnotism and protect the Doctor from electrical shocks. (DW: The War Machines)

However, after the Doctor's first regeneration, the ring no longer seemed to have any use. (DW: The Power of the Daleks) However, this may been out of personal choice during an incarnation. In his following regenerations, none of the Doctors tried to resume wearing the ring, leading to a guess that he had lost it somewhere in the TARDIS.

However, this was not the last that would be seen of the Doctor's ring. 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. (DWM: The Chameleon Factor) The ring was then seen to be worn by the Seventh Doctor in several subsequent Doctor Who Magazine comic stories.