Obsidian is a block that takes 15 to 50 seconds to collect. It requires 15 seconds with a diamond pickaxe, and 50 seconds with anything else. If you are wanting to make a near-indestructible building or structure, obsidian the best choice. Sometimes it is naturally occurring, but often times it must be manually made. First, you will need to find flat lava that is not flowing. Then, collect water with a bucket, and put the water onto the lava. Alternately, find flat water and collect lava. A blackish-purplish block should appear, which is obsidian, and requires a long time to collect. It is much easier to just spawn obsidian, but you will need a mod to be able to spawn it in Single Player mode.