Cut a hole slightly smaller than the gemstone to be photographed, it can be a round hole, square hole, rectangular hole etc, etc, Use a hole punch, or knife edge.

Size the acetate to it fit across the opening in the frame.

Use tape to hold it in place.

The stone will appear to float above any background material you use. The further the stone is from the background (more elevated), the more the background will be out of focus.

(White paper is used in party of this last photo to better illustrate and photograph the set up.)