I'd expect plain black velvet would work rather well, allowing you to capture pretty much just the jewellery in question. (Rich blues & reds can work well too if they complement the piece)
If you have an off camera TTL flash available, this should give you a fair range of options with lighting used either direct (specular) or bounced of white card (more diffuse).
If you'd prefer continuous lighting there are numerous designs of DIY light boxes/tents on the web. (
www.instructables.com has at least 3 IIRC).
Depending on the size of the subjects you might want to get a bit closer in, there's an excellent thread on budget macro if you do.