Need a metal coat and trade like a golem I like Pokemon so much that i almost played every Pokemon game
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In Pokemon Yellow Onix does not evolve, it is a one stage Pokemon.
You can only get an "onix" in Pokemon sapphire if you have a Pokemon version that has an onix then use a linking cable with other GBA to trade it and then you'll have an onix but "onix's" data can't be stored in your pokedex.NOTE:You can only trade with a Pokemon version that have the same region as your Sapphire has;HOENN REGION.
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