Obsidian

Obsidian is a deep purple block that was first released into that game at version 0.30. Obsidian is not found naturally in normally generated maps, except in Infdev.

Obsidian is available for use in creative mode. It is mostly 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 magma, when a water spring and a magma spring collide it creates cobblestone. The obsidian never actually spawns with the chunk, but is created by the environment. If you have a Bucket with water and a magma pool, you can farm Obsidian by pouring the water onto the magma as it is exposed. 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. Obsidian is one of the most resistant materials to TNT, which gives quite random and little damage, which makes it one of the best material for protecting your valuables, along with cobblestone, mossy cobblestone, and possibly natural stone. Obsidian takes the longest to destroy without TNT.

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: