Ryan: Thank you. That's what I was starting to suspect...
Peteski: Good question. Rock molds are easy to make, but I've never tried casting parts. My secret ingredient for rock molds was used dryer fabric softener sheets. Put a few layers of latex on a wet rock, wait for them to almost dry, lay a sheet over them, and add some more. For a big rock, use more than one sheet. For a more durable mold, repeat the process, layering latex and sheets, making sure the latex soaks through each sheet.
My molds were on a cubic chunk of coal about eight inches on a side, for sedimentary cliffs.