Generally it looks good with a seamless plain white or velvet black background - so get some thin white card or some good quality black velvet fabric and curve it within your tent. An alternative is to use a piece of transparent plastic on top of a black or white card which gives subtle reflections.
For rings you can fix them in place with a very small blob of wax or tack. Earrings or pendants can be held on some transparent wire hanging from loops on your light tent.
Your camera should be on a tripod, with mirror lock-up enabled and using a remote trigger of some kind or a timed release. Use a small aperture (large f-number) to give appropriate depth of field. If you need greater depth of field you might want to consider focus stacking, although this can be difficult to get right without additional hardware like a focusing rail.
I would suggest placing your lights in different positions on the outside of your tent and experiment with what looks good. Try using one of the lights on a higher power to over-light the background to give the pure white, while the other lights the subject.
For reflective objects, try using sections of white and black card standing vertically around the subject, to give pleasing white/black transitions.
Clean the jewellery very carefully. Any scratches or dust will show up and will need to be removed in post-processing, so save yourself some time by cleaning as much as possible to start with.
To get sparkle you can try using an additional light just below the camera - adjust as necessary to get the refraction that makes diamonds and other jewels look great.
There are also plenty of good jewelry sites on the net for inspiration.