Yes, exactly what I mean!
Usually, in Rhino, I draw a circle with the given diameter and then create a form; moving some points I try to achieve the model I need and use the circle as reference. So I try to be nearest to the circle, it doesn't matter of the precision, because when the ring is casted and polished it change its dimensions.
The important things are the setting for the stone that must be precise... but I can use nurbs with standard commands.
So, yes... it's good as you wrote!
I think that, starting from a cube, I can add faces following the guide circle to form a ring... right?

Thanks
Marco