When I went looking for things to embed in the resin I found a lot of pieced of broken jewelry that I have found in parking lots over time. One I liked a lot was a filigree piece with set stones.
I cut scrapbook paper for a background and pressed it into the resin in the mold. Then the scrap piece was added and resin poured in to cover it. Doming was done after the piece was cured and removed from the mold.
I have no idea why these gems look right when the self-adhesive rhinestones do not.