I take it your going for a silver or platinum ring?
A lot of the make or break part of this is going to some down to the material settings and the rendering that you use to try and get the best results.
If I were you I would have a plan on how your going to approach the rendering before you work on the material as either using software or mental ray, (HDR, studio style, etc etc) will effect your over all look. Once you've decided how your going to approach it then I would go onto the materials, a good shiny meatal should take care of the ring, as for the diamonds if oyu have a search I posted a scene that I made a while ago, and also Fallot posted his settings for a crystal material that he was working on.
Hope it helps and post your progress!
"No pressure, no diamonds" Thomas Carlyle