Joe
I agree, but like a lot of projects, practice makes perfect. Try some small pieces first. I know there are products on the market that model railroaders use for making molds of real rocks. It's a latex rubber product that you use with bandage gauze in layers. The molds last a real long time and can be used a lot as long as you use a mold release with them. Just pour your mix in, let it start to harden and apply it to your base material and Voila! It has all the nooks and crannies just like the real thing!
Mike