Minecraft wiki entry for Obsidian, associated text: Obsidian is one of the hardest blocks to destroy in Minecraft. In reality, obsidian is glass-like in texture and quite fragile.
Geologically, obsidian is a volcanic glass that forms when lava is flash-cooled. It's the only igneous rock that's glassy.
Other volcanic rocks in the game are andesite, basalt, and tuff.
Destroying an obsidian block with a diamond pickaxe, while underwater, without standing on solid ground, would take 3 minutes and 54 seconds.
71.25 times the explosive power of TNT would be required to destroy a block of obsidian; however, additional TNT increases the blast radius, not explosive power. It is possible to /summon an explosion strong enough to destroy obsidian.