Obsidian

Obsidian can only be mined by diamond pickaxes. Rather than diamond pickaxes, they are merely impossible to break. You could try myth busting but TNT will not work against obsidian. Obsidian are used for 2 things only, Nether portals and Enchantment Tables. When using a diamond pickaxe, the obsidian will be as fast as punching stone so, the point about this topic right now is, don't try fist punching obsidian.

Minecraft 1.0
Minecraft outdated to it's max but it still had obsidian. Obsidian were able to break in one shot with any block. According to updates, they now must be destroyed with diamond pickaxes.