Cobblestone

Cobblestone is a darker block, often looking like a roughly paved area. This block only occurs naturally in dungeons, NPC Villages, strongholds, or when water and flowing lava come into contact. It makes great walls for constructions such as castles, towers and roads. Cobblestone is obtained by mining stone. Cobblestone is slightly harder to mine than normal stone, and there is an alternative version of the block known as Moss Stone, which can only be obtained from dungeons. Silverfish have the ability to enter and hide in cobblestone.

Current Uses
Cobblestone is required to craft furnaces, Cobblestone Slabs, stone stairs, stone tools and weapons, levers, Pistons, dispensers and Brewing Stands. Because of the abundance of stone, cobblestone is a commonly-used building material. Cobblestone can be smelted back into smooth stone in a furnace.

Stone tools are easy to craft and widely available early in the game, making them very efficient even when iron is accessible. However, they posses the lowest enchantibily of all tool sets, surpassed even by wood.

History
Cobblestone was one of the first ever blocks as seen in very early Minecraft footage, when the game was just a clone of Infiniminer, called "Cave Game". Its textures are from one of Notch's previous games called RubyDung. 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 wool and obsidian appeared.

Cobblestone started out with this texture which looks very similar to the Pre-Beta 1.7 one, but with less detail. In Minecraft 1.7 the texture was changed from this, to the current which is lighter and a little smoother. Cobblestone can be crafted into many stone tools, they may not mine gold, redstone or diamond, but they are a very efficient way to mine through just stone.

Trivia

 * Cobblestone is created when water attempts to flow into a block already occupied by flowing lava.
 * This means that infinite amounts of cobblestone can be created by a generator, technically qualifying it as a renewable resource.
 * When water flows into the source block of lava, it creates obsidian instead.
 * Cobblestone is resistant to Ghast fireballs, making it useful for guarding portals.
 * Until the release of Minecraft Beta 1.3 cobblestone was used to create stone slabs.
 * Cobblestone makes up the walls and, along with Moss Stone, the floor in dungeons.
 * In the coding, Cobblestone is referenced as "stonebrick."
 * The current texture for cobblestone has the top and bottom halves swapped and brightened.
 * Although cobblestone is clearly riddled with faults and cracks, it has a higher blast resistance than most blocks and is on par with many others (including stone).
 * You can find naturally occurring cobblestone in dungeons, NPC villages, or places where lava and water flow together.