Obsidian is a block that takes fifteen to fifty seconds to gather. It requires fifteen seconds with a diamond pickaxe, and fifty seconds with everything else. If you are aiming to make a near-indestructible building or structure, obsidian is 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. This 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.