Onix > trade w/ Metal Coat > Steelix
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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. :)
To evolve onix you must give it a metal coat then trade it and it will evolve
There is no such thing as a Crystal Onix. It only exist in the anime.