you find onix at cliff and union cave
Onix > trade w/ Metal Coat > Steelix
There is no such thing as a Crystal Onix. It only exist in the anime.
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.
You must Trade Onix while it is holding a Metal Coat in order to Evolve into a Steelix.
You can find onix in the cave which takes you from the 1st gym town to the 2nd, sorry for not knowing the name, my game is frozen :) (second addition to answer) Go to Violet City...There is a house in the bottom Left of the town. The Nerd with the glasses will give you an Onix if you trade with him...The Onix's name will be Rocky, at it will be at whatever level the Pokemon you traded him was.
Mt. Silver, Victory road......most caves
Trade it whilst it is holding a Metal Coat.
To get a Steelix, making it hold a Metal Coat
You can find an Onix in Dark Cave or in Victory Road in the main game.
Find the crystal onix, make it weak and catch it with your best ball
Graveler 51 Rhyhorn 51 Omastar 53 Onix 54 Kabutops 52
Onix can be found on seven island, six island and rock tunnel.