Obsidian

Obsidian is a purple marked and black block, it is known for its huge blast resistance, being invunerable to most explosions at many powers. The stuff is used to create Nether Portals and Enchanting Tables. Obsidian can only be mined with a Diamond Pickaxe.

In the Overworld, Obsidian never spawns in a chunk, it has to be created with the enviroment when flowing water meets stationary lava. Creating Obsidian like this creates nonrenewable lava.

Natural Obsidian lakes are also found rarely (this is the only time Obsidian spawn with the chunk) they contain a great deal of Obsidian, although mining them can be somewhat dangerous because there could still be remaining lava underneath the Obsidian surface, although with a bit of thinking these problems can be avoided. For example fence of any flowing water rapids since these could possibly push you near to lava. Then start at the edge of the obsidian lake, dig one block down next to the Obsidian edge andplace a water bucket there. Now as you mine the Obsidian any lava underneath the Obsidian will turn to even more Obsidian creating for you eventually another surface to get more Obsidian from.