I can see any dark colour best showing off jewelery better than whites, though it can/has been done in pure white in a few catalogs that I've looked at in the past. Darker colours just seem to give it a much richer/vibrant look.
I just tried out something that I think could be esily used with jewelery photography, to place a little emphasis on the item... its not jewelery, but try it out:
I was playing with selective colouring and instead of making one layer B&W, just motion blur it and bring out the unblurred layer on top. This could be an interesting effect to do with a special piece that you really wanted to make look different, IMHO.
I find the background distracting from the item. The left side of the ring's beautiful design fades a little becuase it reflects the colour of the granit counter top and it doesn't compliment the item.
I think one effect one could do is use a small stand, shallower DOF via bigger aperture and a greater space between the background to subject and a camera close to the subject for more background bokeh. I have to try something a little different for the next experiment.