Earth. bởi the way the main elements r Earth, Water, Fire, and Air (unless u count Spirit as some do) Lightning is a branch of Fire. steal and crystal r off branches of Earth Ice and any liquid (and steam, though sometimes it's confused with a branch of Air. since it's water in a gas form it's still water) is an off branch of Water all other "elements" r really off branches of 1 of the 4 of them. even Darkness and Light is technically a branch of Spirit bending depending on the person's aura. (meaning the aura's màu sắc would determine which he/she could bend) keep in mind this is all based off all the sách i've read and all the shows i've seen.