Minecraft wiki entry for Things_not_to_do, associated text: Obsidian is a blast-proof block which provides great protection but takes a lot of time to collect. The time requirement for collecting obsidian makes it difficult to use in large quantities. Also, despite the fact that obsidian has a high explosion resistance, it can still be destroyed by blue wither skeleton skulls from the wither. If the player is concerned about protecting valuables from explosions, they do so in an efficient manner to use less obsidian.