|$1 wood frame from Michaels Art Supply - acetate
This type of wood gem frame is easy to use, and works well as long as you do not get the camera pointed directly down at the gem with a bright light from the front. The acetate will reflect too much light from around its surface in that type of situation. You can easily produce reflections with this set up. It provides enough acetate real-estate to make several cut-outs and position several gems in the same shot if desired.
You can also use with necklaces and other types of jewelry to help isolate backgrounds. Punch earring posts through the acetate, or use the pin on the back of a broach to affix it to the surface.