A Cobblestone Generator is a structure that gives you cobblestone without harming the terrain. Look at the pictures to know how to make one. Make sure there is a hole infront of the water block, like in the first picture otherwise the water will turn the lava into obsidian.
To make a cobblestone generator, dig a hole 1 blocks down and 5 blocks long. The block farthest along is block 1; closest is block 5, etc. On block 2, put a hole 1 block down. Now, Put water on block 1 and lava on block 5. On block 3, infinite cobblestone will spawn. Mine the cobblestone and more cobblestone will appear!
Note: Put a hopper under block 3 so if the cobblestone falls down, the hopper can collect it.