working at an iSold It shop i have had to photo jewelry before.  Things i have picked up along the way:

1.  Sometimes a more topdown view comes out ok - and that takes away the need for an infinity curve - just have a plain white peice of paper.

2. Lots of lights all around the ring.

3. Put up black screens on one half of the shot and white on the other - especially when photoing silver.

4. A small peice of white-tack to hold up the ring goes a long way with tip #1

And other pointers i have yet to try due to lack of resources:

-Use point source lights and lots of them - it makes the gems sparkle more ;)

-Use a macro ring flash

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography