I have personally used rocks from a local creek. I have chosen granite-based rocks or any other hard rocks (not calcium-based). I have used metal brush to clean them really well, washed them, boiled them, sanitized them with ammonia solution, washed them again and then put together using silicone sealant. Looks great. Now that I found out that my silicone is a mildew resistant kind, I have to re-do all of it because I am afraid of the chemical adverse effect on my fish that I do not have yet.