The Master's ring

The Master's ring was an item of Gallifreyan technology that the Master wore during his incarnation known as Harold Saxon. (TV: The Sound of Drums / Last of the Time Lords) It was a signet ring with a large, green jewel inlaid in silver. After his death and cremation, it was collected by a member of his cult and was used during his resurrection. (TV: Last of the Time Lords, The End of Time)