You can't.
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On mine it claims that Dugtrio evolves into onix whic is kinda weird
diglett evolves into dugtrio, onix is its on Pokemon that evolves into steelix using metal coat.
Dugtrio.
geodude and onix
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 :)