Minecraft wiki entry for Basic_CSGs, associated text: Notes about the schematics here: Stone brick indicates "any block", while gold blocks indicate opaque blocks (relevant to redstone). For a cobblestone generator, "any block" probably implies "fireproof". Water and lava source blocks may be marked with "s" when there is possible confusion. Cobblestone or smooth stone appear only where they form. An "x" indicates a place to stand while mining the cobblestone.