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. (DW: 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, the Master's ring was collected by a member of his cult, and was later used during his resurrection. (DW: Last of the Time Lords, The End of Time)