I agree and would like to say I find a bright white or dark black background is very hard to photograph on. The camera picks the exposure based on what is sees, a lot of dark black will make it overexpose the image (making the jewelry washed out), a lot of bright white will make it underexpose (making the jewelry dark). I have more luck with off white or a dark gray, but that's just me.
I also find the better the photo is when I take it, the better to outcome is. I try to do as little as possible in adjusting the photo. I usually have to lighten it a bit and try to only have to do that 
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Yeah, as I say, I'm not yet especially proficient with my camera, an admission I suspect will not come as a surprise. I typically set it on macro / auto, and let it do its thing, frequently followed by resorting to PhotoGallery to clean up the mess I made with the camera! It's certainly not an optimal system, is it? LOL. One of these days, I'm going to sit down with the camera manual, and figure out how to really use the dang thing. If only I were better at actually taking the photo, I wouldn't have to put in so much time on PhotoGallery! Oof.
I have pretty good luck with certain items on the black background, particularly when I shoot the "straight-on" photo, from directly above. I suppose that minimizes the reflection of incoming window light on the surface of the paper -- and makes the black background look very black. It's when I want to shoot the item at an angle, and I move the camera over to the side of the object, that I run into trouble, which ususally consists of a bit of graying of the black background. PhotoGallery sometimes helps with this, and sometimes, it doesn't, but I've found that, for certain items, even the imperfect shots I get on the black far outshine the ones I get against the white. Using a black background doesn't work for everything, of course, but here's a recent example of the difference between a white and a black background with the same object:
Of course, had I been able to get a whiter-white in the second photo, I'm sure the brooch would have looked nice against it. Actually, it's the white background with which I have the most trouble. I don't think I'm using a bright white background; rather, I think it's a bit of an off-white. However, I've still not yet figured out how to turn out photos in which the object looks as though it's floating in air, an effect I'd very much like to be able to achieve. Instead, even after some time on PhotoGallery, my white backgrounds still appear as a very light gray, an effect I'm certainly not going for! 
I read, somewhere online, about camera white balance, so I watched a couple of YouTube videos on how to set a custom white balance, and tried to follow the steps, but I figure I must've gone wrong somewhere, because my pics on a white background don't seem to have improved appreciably. I also built myself a light tent, but haven't had any more success with that than I did in setting a custom white balance. Oof. Yeah, the photography part of selling clearly is not my strong suit, and I do admire and envy those folks who've mastered it.
