Color keys won’t work. The previous suggestion of using a pure black background is the only way to make it happen. What I would suggest is to mount the jewelry on on a thin piece of glass a few feet off the ground so that you can put (on the ground) just enough black to fill your field of view. Then you can put either colored paper or white paper around that to add to the appearance of the diamond. (note: tilt the glass offset to your lense so that when you use a polarizer filter you can eliminate the reflection of the jewelry and any surranding illumination…. then, using dental wax, mount the jewelry at the proper angle for the lense). Of course I could go on and on about how to light the actual jewelry once the previous work has been done, but I’ll assume you know the rest.
Hope this helps.
Danny