Obsidian farming refers to methods to obtain large quantities of Obsidian. Obsidian does not spawn naturally with the terrain (except in The End), instead being created by natural events or player action.
The simplest method is to pour water over lava source blocks, either in a naturally-occurring lava pool or transported in buckets. A more advanced (and renewable) method is to take advantage of Nether portals being created when no destination portal is in range or the recreation of the 5×5 platform when using the End portal.
In the Overworld lava primarily occurs deep underground, but can sometimes be found in small lakes at or near the surface. Lava springs typically contain only a single source block, which is not particularly useful when the intention is to "farm" obsidian.
Lava is much more plentiful in The Nether, but as water cannot be placed there it is necessary to bring it back to the Overworld in buckets for conversion to obsidian.
Lava drenching is not a renewable method of farming obsidian, as lava source blocks are not a renewable resource.
In-place drenching and mining
The simplest method for obtaining obsidian is to find a lava pool, pour water on it, and then mine it with a diamond pickaxe. Seeming natural deposits of obsidian in the Overworld were created by water falling on lava in this way.
Note that the layer of obsidian thus created will often have more lava underneath, which will frequently destroy the mined blocks before they can be collected. The solution is to mine a single block (which may be a stone or dirt block next to the obsidian slab) and place water in the hole. Then when the obsidian blocks adjacent to the hole are mined, water immediately flows into the space to drench the lava beneath before it can burn the mined block. If you are standing in the water to mine the adjacent obsidian, be careful to move the water source block periodically so it doesn't reach its limit and push you into undrenched lava.
Obsidian structures such as Nether portals may be created in place, without the use of a diamond pickaxe, by placing a layer of lava source blocks in a mold, drenching, and then repeating for additional layers. Dirt or another easily-mineable block is typically used for the mold.
A much faster form of obsidian creation for large structures can be made by building "scaffolding" to create a frame two away from the desired location of the Obsidian and using this to create an artificial waterfall a square away from where Obsidian is wanted. It is then possible to "float" inside of the waterfall and create large amounts of obsidian by placing buckets of lava next to it. This has the advantages of not needing to be built up and have the water moved with every new row, and allows the maximum of 36 buckets of lava to be converted at once. Care should be taken to remain clear of the row where obsidian is being created, however, as the solid block created can hurl the player through the air and cause a large amount of fall damage.
|Obsidian Scaffolding Guide Video (view on YouTube)|
To farm obsidian using Nether portals, travel to the Nether and build a second portal at least 16 blocks away (and preferably not much more than 16 blocks) from the arrival portal. Using this second portal will bring you to a newly-spawned portal in the Overworld just over 128 blocks away from the original portal. Mine out the new Overworld portal (or the old one, if you'd rather). When returning to the Nether, you'll find both portals are still active and using the second will spawn another portal in the Overworld at or near the place where the mined portal had been.
In Multiplayer, a single portal may be used: one player travels to the Nether, then the second destroys or deactivates the portal in the Overworld (this could also be done via some mechanism). The first player then uses the Nether side of the portal to return to the Overworld, which will create a new portal nearby.
Another single-portal method is to travel to the Nether, mine out the Nether side of the portal, stash the obsidian in a chest, and die. When returning to and using the Overworld side of the portal, a new portal will be created in the Nether.
In all cases, be sure to maintain a 4×4 area of flat ground (with at least 4 blocks of air above the entire area) near the location of the destroyed portal, either by filling in the holes left by mining out the base of the portal each time or by leaving the four base blocks unmined. If this is not done, you may suddenly find the newly-created portal is in a cavern underground because there is no longer any valid location for a portal to spawn on the surface. Also, be sure no other portals are within 128 blocks in the destination world or you will be taken there rather than having a new portal created. See Nether portal#Portal Search and Creation for details on the spawning algorithm.
When using an End portal to travel to The End, a 5×5 platform of obsidian is created for you to appear on. If the platform is damaged, it will be recreated. However, getting back requires either having killed the Enderdragon or killing yourself.
Automatic Obsidian Generators
The biggest problem with obsidian mining is the mining part - even if you use the best (no mod) pickaxe, it takes 2.5 seconds to mine a single block. Various methods have been used (and patched) to break obsidian blocks automatically, especially with the appearance of the Wither boss which is capable of breaking obsidian blocks with his attacks.
One of the first designs used the Wither to break portions of the 5x5 obsidian spawn platform in the end, and regenerating it with a sand generator that pushed sand through the end portal situated in the overworld. This method was patched however when the Wither was made to shoot blue skulls that were capable of breaking any block (before that the only way for it to break blocks was through getting damaged and breaking any block in a 3x3x4 area around it, making caging it quite simple with cobblestone generators). The following video is an explanation of how this method works. WARNING: this method works only in minecraft 12w36a and several earlier versions.
|Original Fully Automatic Obsidian Farm Video (view on YouTube)|
Recently a new method of caging the wither has been discovered that once more made an automatic obsidian farm possible. Once again it uses the wither to break portions of the end platform, but the process has been streamlined to deliver 9 obsidian per second consistently; meaning that it is possible to fill up a double-chest in 6.5 minutes. This works from minecraft version 1.5 to 13w38c. WARNING: due to differences between single player and multi-player, this method is only 100% safe in multiplayer worlds; if set up in a single player world, the Wither boss has around a 50% chance of escaping within 10 hours.
|New 9 obsidian per second Fully Automatic Obsidian Farm Video (view on YouTube)|