Minecraft wiki entry for Obsidian, associated text: When a player enters the End, a platform of 25 obsidian blocks is generated for the player to spawn on. This platform can then be mined, and gets regenerated when an entity (only in pre-1.9/1.16) enters the End, which provides another way of obtaining obsidian renewably. Obsidian pillars also regenerate when the ender dragon respawns.