DIY rock

[apepper]

You may want to add sand to that mix as well, the gravel may be too coarse all on its own. You could also try adding some reinforcement such as pvc pipe if the pieces are thin and branching or eggcrate cut to size.

You may have already seen this site but it describes a concrete mix for diy rock. 8 parts concrete, 13.5 parts sand, 11 parts gravel and 10 parts plastic shavings (light reinforcement)

http://www.mindspear.com/reef/concrete/