Minecraft wiki entry for Shelters, associated text: Once you have many stacks of gravel, it can be useful to convert most (leave some for mining) of your gravel into flint. This is just a big empty room (a 4×4×4 space holds a stack of blocks), which you fill with gravel, before destroying the gravel, getting some flint in the process. It is most efficient to do this 10 - 15 times for every 20 stacks, though about 5 for 10 stacks, and 30 for 50 or so stacks. This room may include a chest with shovels, as well as ingredients and a crafting table for quick production of arrows, and flint and steel.