Cobblestone

"Cobblestone, as you’ll know if you’ve played Minecraft for more than about ten seconds, is what you get when you break stone. As a result, it’s one of the substances in the game that will fill up your storage chests faster than a diamond pick chews through sugarcane. The glut of cobblestone you obtain in the course of normal play, combined with its blast resistance, is why it’s usually the favoured building material of anyone who builds for function rather than aesthetics."

- Duncan Geere

"Moss stone was added to Minecraft in the very earliest days of the game in the same update that gave us iron, TNT, bookshelves and obsidian. During the game's Indev phase, the player first spawned in a small house made of moss stone, and later on the block was also used in the construction of jungle temples and zombie villages."

- Duncan Geere

Cobblestone is a common block, obtained from mining stone. Its texture resembles block of stone with a largely cracked surface.

Mossy cobblestone is a variant of cobblestone with moss growing in its cracks.

Both blocks are mainly used for crafting or as building blocks.

Obtaining
Cobblestone requires a pickaxe to be mined, in which case it will drop itself. If mined without a pickaxe, then the mining will be slower and 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
Both cobblestone and mossy cobblestone occur naturally in dungeons, jungle temples, pillager outposts, villages, and underwater ruins. Only cobblestone generates in strongholds and woodland mansions and mossy cobblestone can be found in the form of scattered boulders in the giant tree taiga biome.

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
Cobblestone and mossy cobblestone can be used as basic building blocks, as it is highly blast resistant and fairly common.

Silverfish
Silverfish have the ability to enter and hide in normal cobblestone, creating an infested block.

Repairing
Stone swords, pickaxes, axes, hoes and shovels can be repaired with cobblestone in an anvil.

Trivia

 * If a player were to turn Inverted Colors on, the cobblestone texture will look like end stone, but blue.
 * This is because the textures of end stone and cobblestone are inverted, but don't use the opposing colors matched with the High Contrasting.
 * Looking at the top of cobblestone the texture shows an L. This can be used to find north.