Further to my earlier comments about how cameras underexpose product shots on light backgrounds, the converse is also true about items on dark backgrounds - the camera tends to over-expose - so you need to compensate for that in some way. Exposure is calculated by the camera based on the expected average tonality of most general photographs.
If your background is especially dark or lighter than average, you need to tell the camera that it is and you'd like it staying that way please. This is what the Exposure Compensation setting is for, sometimes called Exposure Value - EC or EV. Compensating for the extra dark or lightness in your subject. You use negative/minus EC/EV - usually to the left/bottom of the scale in your camera settings - and positive/plus EC - the top of the scale for light images.
'Stops' are your units of measure:
The unit of measure for photographic exposure is a stop - a detailed explanation of that term is perhaps beyond the scope of this piece, but I will use the term periodically and your camera manual no doubt refers to it too. Your scale usually allows for something like two stops adjustment in each direction - 2 through 0 to +2. The very quick and dirty explanation is that if you halve or double the light reaching the camera sensor or film plane, you've adjusted it by a stop. So a stop is half as much or twice as much light. If your shutter speed was 1/100 second for example and you changed it to 1/200 - that would be an adjustment of a stop - your shutter is now going to be open for half as long, letting in half as much light - a stop less.
I happened to take some with dark backgrounds today - in an attempt to best show off some especially shiny silver plated items and it reminded me to post something about it here while I had a working example.
In these shots, I took several in sequence trying to prevent the silver highlights from blowing out (disappearing completely to white, with no data and therefore no detail recorded) and was surprised at just how far I had to underexpose to keep the black black and the silver with details in.
I personally tend to use a manual exposure mode and just watch the meter - but for most people, working in an auto or semi-auto camera mode and using exposure compensation might make life easier - and not all cameras have a full manual mode.
These are the results and by which time, the meter was showing them as well over a stop underexposed. I still wanted to retain some of the linen texture of the card background I was using, so didn't allow it to go fully black. The second shot below is rather darker, with almost 2 stops of underexposure. Had I taken the photograph and allowed the camera to set its own exposure, the black card would have been a mid-grey colour - not what I wanted. A further explanation of why this exposure compensation is necessary is on my photography tutorial site. I'll add those tutorials here shortly.
If you like the jewellery pieces, they are for sale (and can be made in other colours) either on Etsy (US Dollars), Folksy (UK/ Sterling) or my own web site. I think the site here darkens the images a little when it resizes, so please do click them to see the larger originals.