I had the same problem with that ring. I mentioned it to my SA and she spoke to the manager and they let me exchange it. I had it for almost a year, but only wore it a few times. I think it might be the copper because I have other wide rings that don't bother me at all.
I'm sorry you experienced the same problem. I always wear wide rings (love the wide Tiffany Somerset ring) that don't bother me at all either. I've only had the Rubedo ring around a month so hopefully exchanging it will not be a problem. Do you experience irritation with any of Tiffany's silver or gold, specifically Tiffany's rose gold? I have never tried their rose gold but now am worried that the copper content in the rose gold may irritate my skin like the Rubedo.