Obsidian

Obsidian is a deep purple block that was first released into the game in version 0.30. Obsidian may be found on rare occasions, when there is flowing Water and still Lava nearby. Obsidian is created when flowing water hits a lava "source block"; when water and flowing lava collide, cobblestone is created. Obsidian never actually spawns with the chunk, but is created by the environment. Obsidian can only be successfully mined with a diamond pickaxe. Each obsidian block takes 15 seconds to mine when using a diamond pick. It can be broken with a non-diamond pickaxe, but this takes 50 seconds and will not yield a collectible block.



Obsidian in the wild is somewhat rare and difficult to mine since it usually rests on top of a Lava block. Obsidian farming is thus the preferred way to acquire large quantities. A basic Obsidian farm is composed of a trough seven blocks long into which seven blocks of Lava are poured using the Bucket. A water block is then placed on top of one end of the trough so that it flows onto the lava and converts it into Obsidian. The lava must be still (not moving) and the water must be flowing into it.

As of the Halloween Update, Obsidian can be used to create portals to The Nether. This requires at least 10 blocks of Obsidian (14 for a full frame) and Flint and Steel.

Current Uses

 * Portals to The Nether
 * Decoration
 * Bunkers. At 6,000, Obsidian's explosion resistance rating is second only to Bedrock.  It is therefore an exceptionally desirable construction material for making defensive structures, in both PvE or PvP, as it is effectively impervious to both lava and explosions.

History
Obsidian was available for use since Classic mode. It is generally used for making dark things, or the black lines of Pixel Art.

In Survival Test mode, Obsidian was extremely tough to break and would drop one or two Cobblestone blocks when broken.

Obsidian was removed in Indev and brought back in Infdev. Obsidian was used to mark the positions of the cardinal directions. This created two intersecting planes extending just above the surface of the solid material that surrounds them. This has since been removed.

Possible Future
Notch has mentioned he may require floating islands to have obsidian in the future:

Later, when asked about making floating blocks fall, he answered:

Farming and Casting
Basic Obsidian Farm

Obsidian can be made by making a pool with 2 squares of water. Then hold your bucket of lava, and right click on one of water blocks. The lava will turn into Obsidian. You need at least two squares of water to make Obsidian. Structures can be constructed out of Obsidian in this fashion by suspending lava source blocks in a mold (usually dirt) and applying water; this method is far easier than mining for obsidian, aside from the distance traveled to relocate the lava.

Obsidian can also be made by making a 1-depth trough with dimensions 2x1. Place a block/bucket of lava into one of the two open blocks, then place a block/bucket of water into the second block, which should have lava pouring (but not filling) the block. The water can be re-obtained using a bucket and the lava will have turned into obsidian, ready to be mined.

Obsidian can also be easily mined by simply harvesting it from portals created by exiting the Nether. Since exiting the Nether creates a new portal each time, all you have to do is use a pickaxe to destroy the obsidian blocks and collect them. This is possibly the easiest method of harvesting obsidian. However, this method is not recommended if your computer is slow to load as it may take hours to receive sufficient amounts of obsidian for large structures.

Safe(er) Mining
Obsidian, if found in the wild, is typically directly above a lava source block, making it dangerous and difficult to mine. This can be overcome, however by placing a water spring block directly over the block you are currently mining. This will convert any lava around the obsidian block into more obsidian, so it's less likely to fall into lava.

Trivia

 * Even though Obsidian is the second hardest block to destroy, in reality, obsidian is glasslike, made by volcanoes, and is actually quite fragile. The misconception is probably due to obsidian being able to reach a sharpness comparable to diamond while being easier to produce, therefore being a favored material during the Stone Age for use as blades and arrow tips.

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