Crying Obsidian

The mystery block can only be found in the "terrain" folder, meaning that some texture pack creators might have already found this. This block has been added in Beta 1.3 but is not added in the game. This block closely resembles Obsidian, but it still unknown as what it might be. It can also appear to be some sort of ore. The block also slightly resembles Netherrack. http://twitter.com/#!/jeb_/status/40350003591380992 Source From http://www.minecraftforum.net/viewtopic.php?f=3&t=192118