Repetable mold
Another method that you could possibly try would be to make a flexible silicone mold.
I used to do this when I played with making my own indoor climbing wall.
I never tried it with cement though (used bondo)
What you do is make a model of what you want out of plasticene or playdoh, then get some silicone and make a number of thin coats allowing each one to dry. after a few coats you should have a flexible mold that you can peel off.
The downside would be that unless you made a bunch of molds your rocks may get boring
Joel
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