Here's how to make a hollow rock.
Start with a wire frame in the shape of the rock you want to end up with. The first frame can have a wide mesh, say 8 inches.
Wrap the first frame with chicken wire mesh.
Stuff the wire frame with trash bags full of packing peanuts or something similar - this will support the concrete while it sets up.
Mix your concrete - you can use some peat moss in place of sand to lighten it up. Or mix it regularly. I used one concrete to four sands in my mix. Keep it thick, not a lot of water.
Press the concrete into the mesh. Your stuffing will keep it from falling through, and your wire will keep it from sliding down. Start on the bottom, and work your way up.
When the wire mesh frame is covered with concrete, cover the whole thing with a sheet of plastic. Smooth the plastic out with your hands, rubbing it some so that the fine concrete raises to the top.
LEAVE IT ALONE until tomorrow.
The next day, you pull off the plastic. The cement is soft enough at this stage that you can carve it if you need to do that to make the rock shape right. If it's fine, then just wet it down and cover it back up for a few days to let the concrete cure.
Painting your Rock it what makes it real
Start with the medium color. Use brownish gray, this is an easy color to get if you have spare paint laying around. Mix one part latex paint with three parts water. Put it into a hand sprayer like you would spray weeds with. (the kind with a pump) Pump it up, and start spitting that paint on your rock. Layers, Layers, Layers. Each time, change your color a bit. You may want gray then brown then beige then black then blue then..... If you look at a rock, you'll see how many colors are there. Put your beige on last - that will be the finishing touch.
When you're done, you have a hollow rock. But it's kind of heavy.
