There are a variety of places you can find an Onix in these games, the most common being Iron Island, Oreburgh Mine, and Victory Road. In Diamond and Pearl, you can also find them at Snowpoint Temple and Stark Mountain. In Platinum, you can also find an Onix at Wayward Cave., You can get a Metal Coat from Byron on Iron Island after completing the National Pokedex. They're also held by wild Steelix, Beldum, Bronzor, and Bronzong., Onix will need to be holding this item in order to evolve when traded., You'll need to find a trading partner to trade your Onix to so that it can evolve into Steelix. You can trade with someone playing Diamond, Pearl, Platinum, HeartGold, and SoulSilver., You'll need to be in the same room in order to trade:

Each player needs to go to the second floor of a Pokémon Center in their game.
Talk to the person in the center of the room to enable the wireless communication and enter the trading room.
Talk to the player that you want to trade with to start the trade.
 When prompted to select which Pokémon you'd like to trade, select Onix. The Onix will evolve into Steelix as soon as it is traded., You'll need to start another trade and have the other person trade you back your new Steelix. Follow the steps above to perform another trade.
Get an Onix.
Obtain a Metal Coat in Diamond, Pearl, or Platinum.
Give the Metal Coat to Onix to hold.
Find someone to trade with.
Start a trade with your trading partner.
Trade Onix to the other player.
Have the other player trade you your Steelix back.
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