Moss Stone, also known as Mossy Cobblestone, was first introduced in the Survival Test and remains in all later versions of the game. It looks similar to Cobblestone, but with moss growing in its cracks. They primarily compose the floors of Dungeons as well as Jungle Temples, which are the only legitimate way of obtaining the block in survival mode. Because of this, many jungle temples and dungeons will be completely mined out, "ruining" their appearance on servers because players are bent on collecting them for trading or building.
Unlike standard cobblestone, moss stones cannot be smelted. They can, however, be crafted into a Mossy Cobblestone Wall. Their only use is in construction and they are often used in combination with normal cobblestone for decoration purposes, such as to make buildings look older or to make Nether Portals more eerie.