An outside shot, hit Goodwill (or your local second-hand place) or large dollar store and check their glassware. A gaudy candy dish upsidedown could pass as a large gem and would have a low profile. Back it with foil for more sparkle.
You could also look for a pre made mold and cast one yourself from Clear Resin. For color I have used simple food Color and it worked well. Or find a nice Glass Gem and then make a mold of it for casting. I agree Keep the Dangerous stuff away from the Drunk People. For the Pre Made stuff look in hobby stores for their Casting isle and sometimes you can find pre Made molds for things like Paper Weights. Also I have found some nice gem type molds with the Soap making Kits. Hope you find what your looking for.