You can make a planter with hypertufa to replicate lava. Use a damp sand mold and work in reverse. A big trash can or bucket filled with damp, tamped sand, excavate the shape of your rock, start filling your sand mold with hypertufa and as you go up the walls use pieces of styrofoam, partially blown up balloons or plastic bags to make the crevices. Just remember that you're working in reverse. If you have peat moss leave it kind of lumpy and the lumps of peat moss can be washed out when you unmold. If you unmold early enough you can use an electric drill and a masonary bit to drill holes into the uncured surface. I think there's a picture on this site of something similar, a planter made for "hens and chicks". The planter can look a little odd at first until the plants start to grow and cover it. Hope this helps. Billie