Minecraft wiki entry for Best_biomes_for_homes, associated text: Basalt deltas are a unique biome in the Nether, as netherrack rarely generates within it (aside from structures). Instead, basalt and blackstone make up most of the terrain. Small lava pools are dotted around the biome, and magma cubes spawn much more commonly here than elsewhere, with the rare ghast being the only other naturally spawning mob.