A cobblestone generator is where you have flowing lava and water meet to create cobblestone or stone depending on how they meet together (if they meet horizontally cobblestone is created and if they meet vertically stone is created.) Here is how you make one (picture below). If you don't have a hole for the water to flow into, the water will flow into the lava and create obsidian. This gets rid of your lava which is kind of hard to find. You can dig out a hole 1 block deep next to the place where the cobblestone is made so that you can collect the cobblestone right as it breaks, although sometimes it burns in the lava. Also, there is a picture on how to make a simple stone generator. The top picture is the cobblestone generator and the one below it is the stone generator. Ignore the stuff in the background because it's just a bunch of random things I've built. You don't have to use glass because I just used it so that you could see what it looks like better. These generators can be used to make infinite cobblestone for building things and crafting stone tools.
Below is a video on how to make a very efficient, block detecting cobblestone generator.