Crying Obsidian

Crying Obsidian refers to a leaked, abandoned project by Jeb to implement a change-spawn obelisk.

History
This texture was added in Beta 1.3 but was never used in the game. It closely resembles obsidian, and also appears to maybe be a bedrock texture modified to look like Obsidian. It is also known as Bleeding Obsidian among many players for its appearance.

In Beta 1.5, the texture slot for Crying Obsidian was replaced by a grayscale replacement of the side of grassy dirt blocks, which is used for biome-compliant grassy dirt block sides.

Its original purpose was to set a new respawn point, but was abandoned after the introduction of beds. It would have been crafted with an obsidian block and Lapis Lazuli.

On February 9th, 2012, Jeb was tweeted with a request for the return of Crying Obsidian (or more colored blocks). He responded "As soon as I've made preparations for more texture space," suggesting the return of Crying Obsidian might be possible. "i'd say Notch has to fix the Crying Obsidian sometime"-minecraftman2233

Trivia

 * The Crying Obsidian texture still exists in the Minecraft Pocket Edition terrain.png file. (both platforms)
 * It also looks (very closely) like a Obsidian block overlaid with the ore part of Lapis Lazuli.
 * Crying Obsidian was also referred to as the Mystery Block, the Teleblock, and Spawnstone.