Minecraft wiki entry for The_second_day, associated text: Once you have coal/charcoal to spare, you might want to use your furnace to smelt some cobblestone back into gray stone. With this, you can fill holes in underground caves without leaving unsightly patches of cobblestone. You can also make various decorative blocks such as stone bricks, which lets you build more attractive houses and other structures. Any variation of stone bricks are just as strong as gray stone or cobblestone.