Onix resembles a giant chain of gray boulders that become smaller towards the tail. There is a rocky spine on its head and a pair of black eyes right beneath it. This Pokémon has a magnet in its brain that serves as an internal compass. Its body absorbs many hard objects, makings its body very solid. As it grows older, it becomes more rounded and smoother, eventually becoming similar to black diamonds.
Onix tunnels under the ground at over fifty miles per hour, which causes tremors and a terrifying roar that echoes a long way. The massive, winding tunnels it leaves behind are used as homes by Diglett. Onix loves to eat the large boulders it encounters while burrowing underground. As seen in the Pokémon Stadium series, Onix is capable of rotating its head, or any part of their body, a full 360 degrees. It is a very aggressive Pokémon that will constantly attack humans and Pokémon when it is in pain, as seen in the anime. Onix can be found in mountains and caves, though it prefer underground tunnels with little light.