Though Stone occurs naturally in the world, when mined will turn into Cobblestone. You can find stone in great abundance beneath the surface of the world, below the Dirt, and sometimes on the surface itself as well. A Pick Axe is the most efficient means of mining stone, but you can mine it even without any sort of tool. However if you do mine it without a tool you cannot collect cobblestone. When Cobblestone is heated in a furnace it will turn into stone, a more fine version of cobblestone. It is grey and completely smooth. It has the same strengths as cobblestone. It is easier to mine than Cobblestone.
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