Cobblestone

Cobblestone is a darker block, often looking like a roughly paved area. This block only occurs naturally in dungeons. It makes great walls for constructions such as castles, towers and roads. In the survival and indev/infdev modes, you obtain cobblestone by mining stone. It can also be formed by water and lava coming into contact with each other. Cobblestone is slightly harder to mine than normal stone, and there is an alternative version of the block known as Mossy Cobblestone.

History
Cobblestone was removed in 0.0.19a, but returned in 0.0.20a. It was removed due to lack of room in the UI. Because of its dark color it was often used in pixel art to represent darker colors until other, darker blocks like black cloth and obsidian appeared.