it cannot evolve in that game... sorry.
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Trade onix while it is holding metal coat
you have to trade it when held with a metal coat.
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
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.