Onix evolves into Steelix, but it doesn't evolve by leveling up. You either need to trade for it, or catch one at Iron Island or Victory Road.
Hope I helped.
Level: Onix Evolves By Trading.
Trade Onix with the steel coat item to evolve it.
level it up
To evolve onix you must give it a metal coat then trade it and it will evolve
You need to give ONIX a METAL COAT to hold. Then trade him and he will evolve into a STEELIX.
no that is unposable crystal onix is a myth will in all games except one Pokemon cyrestal
give it a metal coat and trade it
When you trade it while it is holding a metal coat.
You find it in Orbegugh Mnes or on Iron Island
all the Pokemon on route 210 in Pokemon diamond and pearl are ponyta, geodude and if your really lucky you can find onix.
graveller, golbat, machoke, medicham, onix, steelix, floatzel
You have to make the following Pokemon hold the metal cloak and then trade them: Onix = Steelix Scyther = Scizor