A signet ring was a type of ring which featured some form of emblem.
The First Doctor wore a signet ring consistently throughout his life. (TV: An Unearthly Child - The Tenth Planet) This ring possessed a number of unusual properties. It was lost by the Second Doctor, but later recovered by the Seventh. (COMIC: The Chameleon Factor)
The Saxon Master also wore a signet ring, 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)