There is a way to manufacture massive amounts of cobblestone with just one bucket of lava and one of water. You basically pillar straight up and dump the lava just next to and below the block you're standing on. The you wait till the flow reaches the ground then scoop up the flow spring again and dump the water on it. As the water overtakes the lava it turns it to cobble stone. If you keep doing this down the line and remember to collect the lava and water every time you can easily make very tall walls this way. It's a HUGE time saver!