Obsidian

Obsidian is made when water and lava collide. It is used in the production of Obsidian Skull, Obsidian Brick, and Hellstone Bars. As of the current version obsidian is also immune to bomb explosions, but not to Dynamite explosions. The easiest way to collect obsidian is to lead a lot of water into a lava pool and mine underwater. Another way is to use the Earthwelling technique. A Nightmare Pickaxe or Molten Pickaxe is required to mine Obsidian. Obsidian is not destroyed by Lava, despite not being of blue quality.

Obsidian generator
Obsidian can be harvested by creating a generator. A generator is an exploit that allows the infinite creation of lava and water, to create obsidian. Liquids in Terraria can be duplicated in this way because of a game mechanic in which a full bucket will always produce on full block of liquid, while less than a block of liquid is required to fill the bucket.

This video contains a detailed explanation on how to set up the machine.

Obsidienne