. Obsidian is the only block that must be mined using a diamond pickaxe. Any lesser tool will destroy the obsidian if you try to mine it. Dig down almost to the bottom of the map and explore. It shouldn't take long to find a large pool of lava. Since you have a diamond pickax, you can turn the whole pool into obsidian at once, instead of transporting the lava in buckets. Create a small wall on one side of the pool, leaving room to put down a water block. This will reduce the chances of water pushing you into the lava. Put the water block down inside the fenced area, one level higher than the lava. It should flow down and turn the surface of the lake into obsidian. Stand at the edge and dig one block deep into the obsidian. There might be another layer of lava beneath it. If you're not careful, you may fall into this lava, or the obsidian bock item will fall before you can catch it, and burn up. If there is lava underneath the obsidian, stand next to the water and mine the obsidian at its edge. The water should rush in as you mine, turning the next layer into obsidian before the lava can cause any damage. Continue to mine as much obsidian as you need, moving the water when necessary.
Obtain a diamond pickaxe Locate a lava pool. Fence off the area. Pour water over the lava. Test the edge of the obsidian. Direct the water to flow where you mine.