The Master's ring

The Master's ring was an item of Gallifreyan technology that the Master wore on the third finger of his right hand during his incarnation known as. (TV: The Sound of Drums / Last of the Time Lords)

It was a signet ring with a large, green jewel inlaid in silver. After the Master's death and cremation, it was collected by a member of his cult and was later used during his attempted resurrection. (TV: Last of the Time Lords, The End of Time)