Crying Obsidian

Mystery block refers to an abandoned project by Jeb that can only be found in the form of a texture in "terrain.png". This texture has been added in Beta 1.3 but is currently not used in the game. It closely resembles Obsidian, mostly due to the matching colors, but it is still unknown as what it might be. It also appears to be a Sandstone texture modified to look like Obsidian. There is no indication yet as to whether this texture will be implemented in the future.

It is also known as "Bleeding Obsidian" or "Crying Obsidian" among many members for its appearance. http://www.minecraftwiki.net/images/1/17/Bleeding_Obsidian.png