Minecraft wiki entry for Cobblestone_farming, associated text: A lava stream touching a water stream is the simplest type of generator. In a 10 block long trench with sources at either end, the cobble will form next to the lava. With a little more digging, you can manage this more compactly, and even get a current to wash the mined cobblestone away from the lava. This and the next design are easily expandable for multiplayer use.