How to get obsidian in Minecraft?

Put water over lava. Simple.

Place a bucket of water next to bucket of lava. When they flow into eachother and mix, it creates obsidian. You can only destroy an obsidian block with a diamond pickaxe in survival mode.

Dump a bucket of water over lava. Then put the water back in the bucket. You'll need a diamond pick to mine the obsidian.

First you are going to need a bucket, a diamond pick, and a source of *still* lava. Lava waterfalls will not work, even if dammed.

1. Fill your bucket with water. This is easiest done on the surface where water is more common.
2. Head to your lava lake, and try to find a place where you won't get pushed into more lava or off a cliff if you get pushed.
3. With your bucket in hand, right click open space immediately *over* the lava. Do not click no the lava itself.
3. *Immediately* right click with the bucket. This will pick the water back up before it has a chance to do any damage.
4. Make sure to place torches down in the now darker area.

You should now have a sheet of obsidian. Be warned that this layer is only one block thick so if there's more lava underneath the obsidian the blocks you are mining might drop into the lava instead of into your inventory. I recommend heading to the edge of the field, digging down one block, and mining the obsidian from the side. It reduces the chances that you will fall into the lava yourself.