Mossy Cobblestone is almost the same as normal Cobblestone, the only difference being that it is covered in moss. It is often used in combination with normal cobblestone to make buildings look old and like ruins. In Alpha mode, it is currently only found in underground Dungeons, though it can be mined and used like normal Cobblestone. However, unlike standard Cobblestone, it cannot be smelted into anything at this time. Notch has said that moss will grow on cobblestone, and there is speculation that it will spread to other cobblestone blocks, making it act similarly to Grass.