Hmm Saruman's Ring? Did he have one? I am struggling to remember. The Three Elven Rings went to Cirdan (then Gandalf), Gil-galad (then to Elrond) and Galadriel.
Now you mention it I do remember something about a ring of Saruman's....help me out Filli!
__________________
Utúlie'n aurë! Aiya Eldalië ar Atanatári, utúlie'n aurë! Auta i lómë! Aurë entuluva!
Saruman's ring, mentioned by Gandalf at the Council of Elrond is part of Gandalf revealing to the Council the corruption and treachery of Saruman. The fact that he had one (a ring) at all is significant. Saruman in his arrogance reveals his treachery in several different ways. Related to his ring he says this; "For you have come, and that was all the purpose of my message. And here you will stay, Gandalf the Grey, and rest from journeys, For I am Saruman the Wise, Saruman Ring-maker, Saruman of Many Colours!" 'I looked then and saw that his robes, which had seemed white, were not so, but were woven of all colours, and if he moved they shimmered and changed hue so that the eye was bewildered. ' " I liked white better," I said. ' "White!" he sneered, "It serves as a beginning. White cloth may be dyed. The white page can be overwritten, and the white light can be broken." ' "In which case it is no longer white,' said I. " and he that breaks a thing to find out what it is has left the path of wisdom." ' " You need not speak to me as to one of the fools that you take for friends,' said he. "I have not brought you hither to be instructed by you, but to give you a choice..."(The Lord of the Rings, The Fellowship of the Ring , Book Two, Chapter II - "The Council of Elrond", pgs 258 - 259) I think that Tolkien mentions Saruman's ring not because it is one of power but rather to show that Saruman has violated the purpose that the Valar have sent him to Middle-Earth for. And that is to organize and lead the peoples of the earth against Sauron. Tolkien writes this to show that Saruman is becoming an ally of Sauron.
I did re-read the "Voice of Saruman" with an eye toward powers of his ring. I think the ring only had power to change the colours of his robes; that his ring is as fake as Saruman is.
__________________
Vocatus atque non vocatus, Deus aderit Called or uncalled, God is present
As it says "He wore a ring on his finger", leads me to think that it wasn't overly significant unto itself, otherwise Gandalf would have said "He wore his ring of his finger", isolating it as a special ring of Saruman's, rather than just 'any old ring', which the quote seems to indicate it is.