"The Master's ring" is a title based upon conjecture.
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The Master's ring was an item of technology that the Master wore on the ring finger of his right hand whilst posing as Harold Saxon. (TV: The Sound of Drums / Last of the Time Lords)
It was a signet ring with a large, green jewel inlaid with an ornate silver insignia resembling the logo of Lazarus Laboratories. After the Master was killed by his wife and cremated, it was collected by a woman (TV: Last of the Time Lords) named Miss Trefusis, a member of the Disciples of Saxon, and was later used to enable his resurrection. (TV: The End of Time)