Obsidian

Obsidian is a nearly black block that has no use except for it's decorative and durable value.

Obsidian can never be found in any generated map other than when pre-created water flows onto pre-created lava in infdev, but never spawns with the chunk. It was originally thought that obsidian was cooled lava, which could emerge if it collides with water, however stone is actually created. Obsidian is only formed when water flows onto a lava spring. It takes 50 seconds to destroy with a non-pickaxe and 15 seconds to destroy with a diamond pickaxe which yields one block of obsidian.

In Survival mode, obsidian takes longer to mine than any other material. Sadly, your only reward is obtaining a few blocks of cobblestone.

In one version of Indev, obsidian blocks were used to mark the positions of the cardinal directions. This created two intersecting planes extending just above the surface of the solid material that surrounds them. Now, it appears they have been removed, replacement is unknown.

Notch has mentioned he may require floating islands to require obsidian in the future: