Whether it's a gem or a metal, if the photo looks black, it's not a product problem, but the photography and lighting skills are not done well.
If you shoot jewelry, you'd better have a light box and shoot it with diffuse light. If you shoot with reflective method (flash), it is likely that the product will be shot in black, because when it is highly reflective, the reflective place will emit bright white light, and the camera will automatically adjust to darken other places, and the contrast will be automatically optimized where it is highlighted. So if you want to capture the taste of primary colors, you just need to strengthen the surrounding light sources, and the camera does not use flash. If possible, it is best to have a light box. If not, you can surround it with a few pieces of white paper, put a white light on it, and the camera will shoot it from the front.
The above is my opinion, I hope I can help you. If it helps, please choose my best answer, and I will continue to answer your follow-up questions.