The Master's ring

A signet ring was worn by on the ring finger of his right hand whilst he was posing as Harold Saxon. (TV: The Sound of Drums / Last of the Time Lords) The Tenth Doctor later recognised it on sight as "the Master's ring", and discovered that it was part of a contingency plan for the Master to return from the grave. (TV: The End of Time)

Description
It was a signet ring with a large, green jewel inlaid with an ornate silver insignia consisting of several interlinked circular symbols. (TV: Last of the Time Lords) This symbol was used as the logo of Lazarus Laboratories, (TV: The Lazarus Experiment) whose research was funded and overseen by "Harold Saxon". (TV: The Sound of Drums)

History
After the Master was killed by his wife Lucy Saxon and cremated, the ring was taken from the ashes by a woman (TV: Last of the Time Lords) named Miss Trefusis, a member of the Disciples of Saxon. According to the Tenth Doctor, it contained "part of [the Master]" which thus survived the physical death of his body. The Disciples used the ring, the Potion of Life, and the Master's imprint as extracted from his wife Lucy, to effect his actual resurrection. (TV: The End of Time)

Behind the scenes
As early as the played by Roger Delgado, the Master was depicted as wearing a signet ring on his left hand (TV: Terror of the Autons, The Sea Devils, and others). However, it was never focused upon, making it unclear if the ring later given greater importance by Russell T Davies as a medium for 's resurrection is meant to be the same ring.

Notably, the Master being burnt to ashes except for his distinctive signet ring, which is picked up by a disciple and later used in a dark ritual for his resurrection, echoes the method of Count Dracula's resurrection in several Hammer Horror Dracula movies.