Mossy Cobblestone

Moss Stone (previously known as Mossy Cobblestone) was first introduced in Survival Test and remained in all later versions of the game. It looks almost the same as Cobblestone, the only difference being that it is covered in moss. It is often used in combination with normal cobblestone to make buildings look old and like  ruins. It is currently only found in underground Dungeons, though it can be mined and used like normal Cobblestone. However, unlike standard Cobblestone, it cannot be smelted into anything at this time. Moss Stone is almost universally known as mossy cobblestone, and it is possible that the name was only changed because of a length limit to the popup tooltips.

Notch has stated that moss will grow on normal cobblestone in the future, and there is  speculation that it will spread to other cobblestone blocks as well, making it similar to how Grass grows on Dirt. This is a low priority task, however.

Moss Stone looks very similar to Netherrack in that both blocks are similarly textured, with Moss Stone being grey with green moss and Netherrack being bright-red with dark-red moss.

Bemooster Pflasterstein