you could also have an archer's bump, or whatever they're called, added to the center of the OUTSIDE of the ring, opposite the pearl - i have made a few rings this way, and the way they keep your ring from turning is that your thumb has something to fiddle with all day long, keeping the ring in place. of course, this requires you to have figget-y hands and fingers, but i found that it worked.
of course, i gave them away, i will have to go find another online example for a photo..