Obsidian

Obsidian is a black and dark purple block. It has a high blast resistance, second only to Bedrock. It can only be mined by using a Diamond Pickaxe.

After Update 0.12.0, it will be used to make a Nether Portal.

Obtaining
Obsidian can spawn underground naturally when flowing water meets a lava source block. It can also be created by the player by a similar method. It also naturally occurs in Village chests.

Crafting
After Update 0.12.1, Obsidian will be used to craft Enchantment Tables.

4 Obsidian + 2 Diamonds + 1 Book => 1 Enchantment Table.

Trivia

 * An Obsidian Block takes about 7-8 seconds to mine with a Diamond Pickaxe.
 * Before 0.5.0, the structure created by activating the nether reactor was made out of obsidian but was later changed to Nether rack. Now, only the middle is is obsidian.
 * Its blast resistance is 6000, making it the second most blast resistant block. This means tent and creepers can't damage it.