In the dark cave or victory road.
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catch it
Find the crystal onix, make it weak and catch it with your best ball
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.
There is no such thing as a Crystal Onix. It only exist in the anime.