Minecraft wiki entry for Cobblestone_farming, associated text: When producing cobblestone, one must be careful not to let the flowing water touch the lava source block. Doing so will destroy the lava source, converting it into obsidian. A basic understanding of fluids is helpful to prevent this. If you place a stair into the water source (flat side to the lava), this will prevent the water from flowing over the lava source, but the cobblestone will still form where flowing lava touches the waterlogged block. Placing a slab over the lava will help prevent the cobblestone from being destroyed by the lava, as well as keep you from falling into it.