Stone

Stone is a type of block introduced very early in the game. It uses the same texture on all six sides of the block. Stone makes up a very large portion of a normally generated map: under a dirt layer, a stone  layer can extend all the way to to the bottom of a map (and in  Creative mode, to the edges). In the stone layer, Caverns, Ore and Dungeons (Alpha only) can be found.

Stone is often used in Creative mode for making roads, paths and to make walls of stone for dungeons or tunnels.

In Alpha mode, stone is a valuable resource. Mining stone with a Pickaxe will yield Cobblestone, which can be used in  crafting or smelted back into stone. Making a staircase mine is a great way to get lots of Cobblestone and is  an efficient way to locate veins.

Stone can be crafted into Pressure Plates, which can only  be activated by stepping on them, and Stone Buttons,  which are activated by clicking on them.

Trivia

 * In very early Minecraft footage, Stone had a Cobblestone texture.
 * Stone has the largest amount of blocks in a normal map.