Iwark, also known as Onix, is a Pokémon species composed of a giant chain of gray boulders that gradually decrease in size toward the tail. It is the tallest Rock-type Pokémon, with black eyes, a rounded snout, and a rocky spine atop its head. As it ages, its body grows smoother and more rounded, eventually taking on a resemblance to black diamonds. An internal magnet in its brain acts as a compass, offering directional guidance and preventing disorientation while tunneling.

This Pokémon tunnels underground at speeds exceeding 50 miles per hour, producing tremors and a loud, far-reaching roar as it moves. It consumes boulders as food and incorporates hard objects it encounters into its body, increasing its sturdiness. The tunnels it carves out later become homes for Diglett. Iwark can rotate its head or any segment of its body a full 360 degrees. It behaves aggressively when in pain, often attacking humans or other Pokémon. During battle, it can detach parts of its body and later reattach or regenerate them without lasting injury. After about 100 years, its body develops diamond-like qualities before it evolves into Steelix. It commonly inhabits mountains and caves.