Cobblestone is one of the most common blocks, dropped after Stone is mined with any pickaxe. Cobblestone can be smelted back into Stone in a furnace with a 1:1 ratio. Common uses for Cobblestone include producing scrap in a Recycler or using the EMC value of 1 to make less expensive items in the Energy Condenser.
Cobblestone can also be used in conjuncture with Automatic Crafting Table to create 6 stone slabs with three cobblestone. Once this has been done, you may condense the slabs for 1 EMC each. To continue this further just repeat the steps. This method can also be used for a higher production of scrap.