you can use anything really, I only do I need in containers do I don't know about big rocks pieces but for containers you can use all kinds of stuff. Small pebbles with some potting soil and perlite is simple. Ive used grated styrophome to help with drainage. I plan to do a new one with a large clear container and use anything from glass, colored sand, colored fish gravel, beads, painted rocks. I don't think it makes much of a difference in a large outdoor rock garden but I would choose rocks with angles and sharp edges for an abstract natrual effect and round rocks for a more symmetrical or designed look.
