Obsidian

Obsidian is a deep purple block that was first released into that game at version 0.30. Obsidian may be found on rare occasion, when there is flowing Water and still Lava nearby.

Obsidian is available for use in Creative mode. It is generally used for making dark things, or the black lines of pixel art. In Survival mode, Obsidian was extremely tough to break and would drop one or two Cobblestone blocks when busted. Obsidian was not available anywhere in Indev. Obsidian is created when a water spring flows onto a still pool of lava, when a water spring and a lava spring collide it creates cobblestone. The obsidian never actually spawns with the chunk, but is created by the environment. It takes 50 seconds to destroy with an non-diamond-pickaxe and 15 seconds to destroy with a diamond pickaxe. Obsidian can only be successfully mined with a diamond pickaxe. Obsidian cannot be destroyed by TNT.

Obsidian in the wild is somewhat rare and difficult to mine since it usually rests on top of a Lava block. Obsidian farming is thus the preferred way to acquire large quantities. A basic Obsidian farm is composed of a trough seven blocks long into which seven blocks of Lava are poured using the Bucket. A water block is then placed on top of one end of the trough so that it flows onto and converts the Lava into Obsidian.

Trivia

 * In the first 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 have obsidian in the future: