Signet ring

A signet ring was a type of ring which featured some form of emblem.

History
The First Doctor wore a signet ring consistently throughout his life. (TV: An Unearthly Child - The Tenth Planet, Twice Upon a Time) It did not fit the Second Doctor's finger, which Ben Jackson claimed proved that he wasn't the Doctor. (TV: The Power of the Daleks)

This ring possessed a number of unusual properties. It was lost by the Second Doctor, but later recovered by the Seventh, who decided to start wearing it again. (COMIC: The Chameleon Factor)

also wore a signet ring in his "Harold Saxon" incarnation, which featured an emblem resembling the Lazarus Laboratories logo. (TV: The Sound of Drums) The ring stored genetic information that was later used by a cult to resurrect him. (TV: The End of Time)

Erimem wore yet another signet ring, which bore the sigil of a lioness. It contained a capsule of distilled mandrake root, which she used to defeat Sekhmet. (AUDIO: The Bride of Peladon)

A kryptonite signet ring was mentioned in an issue of Superman, within which it caused the loss of Lex's right hand. (TV: The Return of Doctor Mysterio)