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. Obsidian is created when water collides with lava springs, which cools the lava into obsidian. Cobblestone results when water collides with lava (not a lava spring), rather than obsidian. It takes 50 seconds to destroy with a non-pickaxe and 15 seconds to destroy with a diamond pickaxe. Obsidian can only be successfully mined with a diamond pickaxe.

In one version of Infdev, 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. This has since been removed.

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