While the weather is warm, go outside and scrape up some moss that is on a flat surface. Soak lava rocks (clean ones, like those used in barbeques) in water, wet the moss piece, and wrap it around the rock, leaving some rock surface showing. I tried the old buttermilk and macerated moss recipe that you can paint on rocks outside to get them to grow moss, but inside it just made a wooly, moldy mess too gross to even photograph. Keep the rocks in the shade in a cool place and in a covered container.