Trade it whilst it is holding a Metal Coat.
Onix > trade w/ Metal Coat > Steelix
You must Trade Onix while it is holding a Metal Coat in order to Evolve into a Steelix.
ONIX DOES NOT EVOLVE BY LEVEL! To evolve Onix, equip a Metal Coat and trade it over to another game. Find trade evolutions frustrating? You can just trade any Pokemon for Jasmine's Steelix.
In Pokemon Yellow Onix does not evolve, it is a one stage Pokemon.
In order to evolve Onix in Pokemon Crater you must beat Cynthia. After beating Cynthia she will give you the mental coat to evolve Onix.
Onix doesn't evolve from anything. It evolves into Steelix, but Onix is the base pokemon for that evolution chain. :)
Trade Onix with the steel coat item to evolve it.
To evolve onix you must give it a metal coat then trade it and it will evolve
There is no Pokémon that evolves into Onix.
It doesnt
There is no such thing as a Crystal Onix. It only exist in the anime.
Yes, Onix does evolve into another Pokémon. Onix will evolve into Steelix after being traded while holding a Metal Coat.