Minecraft wiki entry for Obsidian_farming, associated text: To farm obsidian using Nether portals, first travel to the Nether and build a second portal at least 16 blocks away. Traveling through the second portal will bring you to a newly spawned portal in the Overworld. This portal will be about 128 blocks away from your original Overworld portal. You can then mine the new Overworld portal and repeat the process. Each time after the first should yield 14 obsidian.