The magical ring was a popular implement in medieval and renaissance magical practices. The key of Solomon has the designs for a ring which is used to give the magician divine authority. Dee and Kelly's Enochian system also makes use of a magical ring.
Then many other occultists have also had rings with their own personal symbols engraved and consecrated them for personal use. The two above are famous examples from classical grimoire, but in truth any ring can become a magical ring; even one made out of string!