I have gotten my best pics by using a light box,and several halegen (the swirly light bulbs) to light all around jewelry. The next best thing is natural lighting,outside. Flashes are a no-no when photographing jewelry.
It's also important to use a tripod and use your timer,that way you don't shake the camera when you snap the pic and blur your image.
Also a great investment is Photoshop. When a good image comes out a little dark,you can just use the "Auto Color" or "Auto Levels" option and it will brighten your pics to their true glory in just a click. If you happen to know a college student you could get them to buy and get a discount;-)
I have a small cannon camera,nothing fancy,no big lens on it. I have had a couple of photgraphers use their fancy cameras and take bad images because the lighting was off and they didn't use a tripod.
Lighting,tripods and Photoshop,that my advice;-)