And your PB knows you want only 1 more? That seems pretty stinky, but maybe it's more difficult than I realize.
Have you tried calling mmgtx.com and talk to their pebbletec supervisor? If they're no help, I would call the headquarters for pebbletec. I think that's in Arizona. Two years ago they were very helpful to me on color samples.
Another thought or last resort ... For my pebbletec, a pre-inspector (not sure what his position was) came out a day or so before the finish was installed. But there was no call ahead he was coming. He was super nice and eager to listen. If that happens for you, I wouldn't hesitate to ask if they had one gem lying around. Surely they do.
Who's installing the gemz? If it is pebbletec, they would need to bring extra in case of breakage or damage. It seems like your PB would want you to have have extras also!
Another thought: cut out cardboard squares in the correct size. Try placement again. Using the right shape and size (as opposed to rocks) might give you a different perspective.
Good luck!