Are you using a tripod with these pictures, or at least a steady table?
Irregardless of the lighting, which I think Eriol probably is right on, the thing with macro photography is that focusing usually has to be really precise, and an inch difference in movement is more than enough to throw off focus. The basic movements of the hand will throw off focus much much faster in macro than anything else, so you need to either be dead steady, or use a tripod. Since you're talking about photographing fixed objects, a tripod makes a lot of sense here.
If you can control the focusing manually, macro work like this would be a good time to do so, especially if you're using a tripod, where you can then make predictable adjustments in focus to get things right.
My sister actually works at a place where one of her duties is macro photography & digital touchup of jewelry for a wholesaler, so I'll make sure to ask her, since she may have tips on lighting that even I don't know about.