I used the Marco rocks bonding mortar, it worked great with no drilling or acrylic rods or any of that. I tried to separate a couple of rock that were cemented together and the rock gave out before the cement did. Here is a pic of the rock structure I made.
Epoxy puddy sux by itself but have heard using it with super glue it works really well however I did do the rods and it worked great in certain applications.