This is an idea I've been kicking around for months, I finally worked out just how to do it. I used sculpey, food coloring, an Iraqi Nickel, and a magnetic purse hook.
I used a couple of green drops in a dime sized ball of sculpy and lightly blended it to achieve a marbling effect. Then smooshed it in the recess of the purse hook and firmly pressed the nickel into it. It's difficult to make out the imprint in pictures. I then used an exacto knife to trim all excess sculpy above the recessed area, and my fingertips to smooth the border.
Then it was into the oven for a half hour to harden. I used a translucent sculpey so the end effect shows depth and appears superficially similar to a geode. 
Next I used an acrylic paint to coat the coin impression then dabbed it away with a tiny brush and damp cloth, leaving only the depressions filled.
Finally a light coating of gloss on the sculpey, the end effect is very stamp like. It's a bit hard to photograph with the all metal and gloss though.
