Cobblestone

Cobblestone is a common block, used for crafting many tools and items. Its texture resembles an uneven, roughly paved surface.

Obtaining
If cobblestone is mined with any pickaxe, it will drop itself; otherwise, it will drop nothing.

Stone
If stone is mined with a pickaxe that is not enchanted with silk touch, it will drop cobblestone.

Natural generation
Cobblestone occurs naturally in dungeons, strongholds, villages, and jungle temples.

Water and lava
When water and flowing lava come into contact, the flowing lava is replaced with cobblestone. However, if the lava flows on top of the water from above, stone will be created instead.

Usage
Cobblestones can be used as a basic building block, as it is highly blast resistant and fairly common.

Silverfish
Silverfish have the ability to enter and hide in cobblestone, creating a monster egg. If mined with silk touch silverfish will not spawn.

Trivia

 * Cobblestone was the very first block added in Minecraft, but its data value is 4.
 * The original texture came from one of Notch's abandoned projects called RubyDung.
 * The current cobblestone texture came from swapping the top and bottom halves of the third texture, then brightening it.