I like using natural daylight for jewellery, mirrored to an angle that brings out relief you want, and filtered through white paper or similar to prevent too much glare. Colour temperature is critical for precious metals, so I usually sneak in a piece of "known white" material in the corner of the shot to set the colour temperature.
I'm not a professional jewellery photographer though, YMMV.
It's difficult getting that though normally especially if you don't have a professional setup and it's winter like it is now :P the best option I think would be just to get a really powerful light with a white bulb and blast it. A bit crude yes, but I expect it would have the best effect. I actually got this by blasting two lights at it but I think they were quite small and that's why the light wasn't picked up so well by the camera.
It's actually in a light box already, so this is leading me to believe it must be a lighting issue that's causing the dullness and shadows.
Avoid LED and fluorescent lighting, their spectrum isn't complete enough to do precious metals justice, in my experience. Incandescent light bulbs are usually a bit on the red side, but that can be corrected in PhotoShop to some extent. Paper can make the light diffuse to avoid glare.
Okay, I'm glancing at tutorials every now and then on photography and they all seem to recommend continuous lighting, doesn't look hugely expensive so it might be something to save up for so I have a proper lighting setup.