An Extreme Hills is a fairly common biome with green grass with a touch of grey in it and very large hills. Floating islands and massive overhangs sometimes spawn here. It is common for ore to spawn on the sides of them, especially Coal. Extreme Hills Biomes are rather dangerous, as there are many ravines, caverns, and places to take extreme fall damage. Emerald Ore only spawns here and in Extreme Hills Edge Biomes. Silverfish stone can very rarely be found in this biome.