A delusion ring will convince the wearer that it is some other sort of ring, a ring of whatever sort the wearer really desires. 

As the wearer will be completely convinced that the ring is actually one with other magical properties, he or she will unconsciously use his or her abilities of any sort (including those of other magical items available) to actually produce a result commensurate with the supposed properties of the delusion ring. 

As referee, you will have to be most judicious in determining how successful the self-delusion can be, as well as how observers can be affected and what they will observe. 

The ring can be removed at any time.