Instructions
Make a bucket by combining three iron ingots at a crafting table. Craft as many buckets as you can fit in your inventory.
Find a water source. Fill several buckets with water.
Find a lava pool. Lava usually flows in deep trenches and underground caves. Collect several buckets of lava.
Dig a trough that's seven blocks long. Drop one dirt or stone block at both ends of the trough.
Pour one bucket of lava into the center of the trench. Wait for the lava to spread out and fill the entire trench.
Pour a bucket of water onto one of the raised blocks at either end of the trench. Obsidian forms when the water touches the lava. Collect the remaining water with the bucket.
Mine the obsidian with a diamond pickaxe for fastest results. To make a diamond pickaxe, combine three diamonds with two sticks. If you use a different type of pickaxe, it can take several minutes to break down one block of obsidian.