I've been playing around with colors trying to get smoething that I really like. The one in the previous picture was a mix of portland cement, white sand, a hardener, and water. I considered using either fiber strands or wire for reinforcement, but I was concerned about their visual affect. I dont think it is necessary do to the size of the item.
I created the mold using liquid latex rubber. I brushed thin layers of latex on my model rock, allowing each to dry before the next was applied. I guess I put about fifteen layers on it over a few weeks. Then, I put a really thick one on to ensure that the corners and base would be strong enough.
Attached, you will find a pic of the rocks I made so far (and the mold). The real rock is in the upper middle. I am getting pretty good at matching it's appearance. I put the previously photographed rock on ebay. I figured I would place a few, give a few away, and sell a few to recoup my costs.
edit to add the pic. <doh>
Where do you buy the liquid latex rubber to make your molds from?