The only way to get Onix on Pokemon Emerald is to trade from Fire Red or Leaf Green.
Onix is usually in Mt.Moon or the Victory Road.
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Unfortunately, you can't get an Onix in Pokemon Emerald. To have your own Onix, you need to trade it from FireRed or LeafGreen. Hope I helped :)
the word shop gives you a clue.
you can only get it by trading from rife red. :)
Onix isn't available in Emerald but could be traded from LeafGreen or FireRed, where it is very common in Victory Road, Rock Tunnel, and Sevault Canyon.
get an onix, then give it the item metal coat, and trade it to another game. it should evolve