Obsidian

Obsidian is a deep purple and black block known for its high blast resistance, use to create Nether portals and long mining time. It is immune to nearly all explosions, the exceptions being a Wither's blue skull missiles and a Fireball with an ExplosionPower value of about 305 created with /summon.

Obtaining
There are a number of ways to obtain obsidian, including some renewable methods. Obsidian can also be found in village chests in groups of 3 to 7, although this is not practical.

Lava and Water


In the Overworld, obsidian never actually spawns with the chunk, but is created by the environment. Obsidian is formed when flowing water hits a lava source block, and can be found naturally where water from a spring or lake has flowed over a nearby lava pool. Creating obsidian like this consumes nonrenewable lava.

Natural obsidian "lakes" provide a great deal of obsidian, but mining them can be somewhat hazardous, because there is likely to be lava remaining underneath the obsidian surface. However, the danger can be minimized with a bit of forethought. First of all, fence off any water flows so they won't push you around, especially if the water flows to the edge of remaining lava. Then, look for the edge of the obsidian "lake", or at least a convenient point to begin. Dig a one block deep hole on a level area next to the obsidian, or even in it. (If there is lava under the hole, you may lose the block you just mined, but this is minor.) Place the water source in this hole — if there's lava beneath, place it against the side of the hole, and it will solidify the lava. Now mine your obsidian, moving outward from the hole. When you expose lava, the water will quench it so quickly that the lava usually won't even burn your newly-mined obsidian. Continue mining outward from the spring as far as the water will travel, after which you can move the water. This method lets you stand on solid blocks rather than in running water.

It is also possible to just place buckets of lava and flood them with water. This can be used to "cast" structures in place (usually in molds of dirt or such), but it is fairly slow and somewhat hazardous because of having to fill and handle many buckets of lava for a large project. There is also the point that lava itself is not renewable, though large quantities can be found even in the Overworld, let alone the Nether.

End pillars
In The End, solid pillars of obsidian can be found on the surface. It is one of the few blocks not destroyed by contact with the Enderdragon, along with End Stone and Bedrock.

End spawn platform
When a player enters The End, a platform of 25 obsidian blocks is generated for the player to spawn on. This platform can then be mined, and will be regenerated when the player reenters The End, thus providing a renewable source of obsidian.

Nether portals
In the Nether, obsidian does not form naturally, but is created when a Nether Portal is spawned. This portal can then be mined, providing another way of obtaining obsidian renewably. This method takes advantage of the fact that a new portal from either dimension, if it can't find an existing portal in the other dimension, will create a new one. This requires 20 or so blocks of obsidian (or 10 and a working Portal) to start with, but those can be used to create from 14 to 18 more blocks on each round. First, build a portal to the Nether (making sure it's the only portal within 150 to 200 blocks), and secure a base on the other side, making sure you can move around safely. Then, move at least 16 blocks in the Nether from the original portal, and build another portal. Light that and go through it, creating a new portal in the Overworld. You can now mine this portal (destroying it), go back to your original portal, and repeat the process.

Redstone
The “Infiniobsidian” glitch allows converting redstone into obsidian: When redstone wire is next to flowing water, and lava then flows over it, the redstone turns into obsidian. (It will usually be necessary to block the lava from flowing into the water.) Note: This redstone wire must not be powered, or it will be consumed without creating obsidian. Since redstone can be obtained by trading, or by killing witches, this is technically renewable. Note that the lava and water are used as catalysts, but are not consumed normally, in some circumstances the lava can be. This method can also be used to create structures in place. Note: Powered rails, detector rails, and activator rails can also be used, as long as they are unpowered. Note: The “Infiniobsidian” glitch can now be done with string thanks to the 1.3 update that allowed players to place string as tripwire, making string possibly the least expensive way to create obsidian.

Mining time
Obsidian is known as the sole block which can only be successfully mined with a diamond pickaxe. Enchanting the pickaxe with 'Efficiency' will speed up the process (by +30% speed/level). Below are the mining times for each level of enchantment.

This is how the mining time works: Obsidian has a "base" time of 75 seconds to mine. A diamond pickaxe mines at 8 times basic speed, which brings it to 9.37 seconds. But the diamond pickaxe is also the "proper tool" for this block, so without it, mining takes 3 1/3 times the base time, which is 250 seconds (4 minutes and 10 seconds).

Usage

 * Igniting the interior of a 4×5 to 23x23 vertical rectangle made of Obsidian will create a Nether Portal.
 * Obsidian is often used to prevent griefing due to its long mining time and contrasting appearance against most other blocks. Griefers are deterred because of its long destroying time. On the other hand, since its long mining time can make it frustrating to remove, it is also used by griefers themselves to block paths, chests, furnaces, or even fill a small base with obsidian. As long as the griefer has enough obsidian, this can be done in a matter of seconds.
 * Obsidian can be used to build explosion resistant shelters. At 6,000, Obsidian's explosion resistance rating is second only to Bedrock, End Portal block, and End Portal. It has the same resistance as an Enchantment Table, and twice that of an Ender Chest. It is therefore an exceptionally desirable construction material for making defensive structures, in both SSP or SMP, as it is impervious to Lava, explosions, and Ender Dragons.
 * Building and testing of structures used to set off explosives, such as a TNT cannon, or an explosive defense.
 * Obsidian is used to craft Enchantment Tables, Ender Chests, and Beacon Blocks
 * Since obsidian cannot be moved by pistons, it may be used in Redstone circuits and devices relying on this property. However, chests and furnaces have the same property and are easier to obtain.

Trivia

 * Even though in Minecraft, Obsidian is the second hardest block to destroy (next to indestructible Bedrock), in reality obsidian is glasslike in texture and fragile (it is, however, black or blackish in color).
 * The misconception of obsidian's hardness may be due to its use during the Stone Age in blades and arrow tips; While fragile, it can be chipped into extremely sharp edges.
 * Its main component is silicon dioxide (SiO2), more commonly found in quartz. Unlike quartz, obsidian has no crystal structure; it is literally a natural glass.
 * It is produced when a suitable type of lava cools quickly (usually in water). Unlike in Minecraft, in the real world obsidian (being an extrusive igneous rock) is often found above ground.
 * Obsidian portals created by the game always have corner blocks, but these can be mined without affecting the portal. If an exit portal is floating in midair, the game will add two blocks of obsidian to each side along the bottom row, increasing the "free" obsidian from the new portal to eight.
 * Destroying an underwater obsidian block with a diamond pickaxe, without standing on solid ground, would take 3 minutes and 54 seconds. (Without, it takes 4 minutes and 10 seconds, like normal.)  Either way, the player will run out of air long before the block is mined.
 * Obsidian, unlike most blocks, cannot be moved by Pistons (normal or sticky).
 * Obsidian is one of only three solid blocks that can appear naturally in more than one dimension, the others being Bedrock and Gravel.
 * If you switch your language setting to Pirate English, it will name the obsidian block: "Rock o' Tears", referring to the unimplemented Crying Obsidian.
 * Obsidian can be found in groups of 3-7 in chests in NPC Villages.
 * Obsidian is most commonly found on layer 10, where environmental water flows onto natural lava lakes.
 * It is a common misconception that enough TNT will destroy Obsidian. Even though its explosion resistance rating is finite, additional TNT or other explosives will have no more effect on an individual block than only one. (It would take 71.25 times the explosive power of TNT to destroy a block of obsidian.) The exception is Fireballs summoned with custom data.   Note that the "Blue Wither Skull"'s ability to destroy obsidian is a special feature.
 * Notch once planned to make floating masses of land require Obsidian veins to stay afloat. However, he later stated on Twitter that he didn't like the idea anymore, so it's unlikely that this will be implemented.