just a couple of tips from my experience.
you can break big rocks into smaller pieces that are easier to scape and make shapes with by dropping onto concrete, so you don't have the 'big pile of rocks' look
use medium or thick cyanoacrylate (super glue) from LFS or a hobby store to glue rock pieces together quickly. CA glue is strong and sets fast but does not like shock, ie being dropped, it is likely to shatter.
after getting shape you want with CA glue, mould aquarium epoxy around the joints to make even stronger and beefy, the epoxy will support the CA glue when it sets, and the CA will hold everything together as the epoxy sets.