you have to trade it when held with a metal coat.
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Trade onix while it is holding metal coat
In Pokemon Yellow Onix does not evolve, it is a one stage Pokemon.
it cannot evolve in that game... sorry.
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.
There is no such thing as a Crystal Onix in the Pokemon games series. It was only in the Pokemon anime.