Obsidian

Obsidian is a dark purple block known for its high blast resistance and strength, most commonly found when water flows over lava.

Obtaining
Obsidian drops itself only when mined with a diamond pickaxe or netherite pickaxe; otherwise, it drops nothing and takes an awfully long time to mine

Enchanting a pickaxe with "Efficiency" and using a beacon with Haste both drastically decrease the mining time. See the obsidian farming tutorial for more information.

Natural generation
Obsidian generates naturally in all dimensions:
 * Overworld:
 * Obsidian generates in woodland mansions with a diamond block concealed by it. These are found in the secret "obsidian room" and "lava room", on the 2nd or 3rd floors.
 * Obsidian generates as part of ruined portal
 * Obsidian generates in sufficiently low ravines and caves where lava and water meet.
 * The Nether: Obsidian generates as part of ruined portals.
 * The End:
 * Solid obsidian pillars can be found on the End's main island, generating all the way down to Y=0.
 * Obsidian also generates as a part of the end ship in end cities.

Post-generation
Obsidian is created when a Nether portal is generated in the Nether or the Overworld. This portal can then be mined, providing a way of obtaining obsidian renewably.

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 gets regenerated when an entity (only in pre-1.9/1.16) enters the End. Obsidian pillars also regenerate when the ender dragon respawns.

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.

Bartering
Piglins may barter 1 obsidian when given a gold ingot with a chance of 9.43%

Usage
Obsidian is useful for building explosion resistant structures as it has, along with anvils and netherite blocks, a blast resistance of 1,200; the highest of all blocks obtainable in survival mode. Obsidian also cannot be pushed by pistons, and also cannot be destroyed by the ender dragon.

These qualities make obsidian an extremely effective defensive block, and it is commonly used in many bases in multiplayer servers as protection from griefers.

End crystals can be placed on top of obsidian, which can be useful in PvP combat, or just as decoration.

Nether portals
Obsidian can be used to create a nether portal frame, which can then be ignited using fire. The frame must be a vertical rectangle any size from 4×5 to 23×23. Note that the corners of the frame are not required.You can actually gain 4 obsidian by leaving out the corners in making your nether portal, as the game generation always creates its nether portal as a complete rectangle with 14 blocks,while the "budget" way of making a nether portal is using only 10 obsidian.

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Trivia

 * Obsidian is one of the hardest blocks to destroy in Minecraft. In reality, obsidian is glass-like in texture and quite fragile.
 * In real life, obsidian is an extrusive igneous rock like andesite, meaning that they are both formed from lava that has cooled down.
 * Destroying an obsidian block with a diamond pickaxe, while underwater, without standing on solid ground would take 3 minutes and 54 seconds.
 * 71.25 times the explosive power of TNT would be required to destroy a block of obsidian; however, additional TNT increases the blast radius, not explosive power. It is possible to an explosion strong enough to destroy obsidian.
 * The blue wither skull has the ability to destroy obsidian as a special feature; its explosion is otherwise no stronger than a ghast's fireball.