You’ll find that many Pokémon have specific evolution requirements. These exact requirements are in Pokémon Brilliant Diamond and Shining Pearl. If you’re trying to hunt down specific Pokémon, you’ll want to make sure that you understand these requirements. For example, Onix has an evolution requirement if you’re going to evolve it into Steelix. Here’s what you need to know about how to evolve Onix into Steelix in Pokémon Brilliant Diamond and Shining Pearl.
The only way you can evolve Onix into a Steelix is if you have an Onix wearing Metal Coat. You can obtain Metal Coat in several ways. One of these methods is progressing through the game and receiving it from Byron on the Iron Islands. Although, you can also acquire it by capturing a Magnemite in the Grand Underground. However, catching a Magnemite with Metal Coat is extremely difficult.
Once you have the Metal Coat, the next step is to trade Onix to another player you trust. During the trade, Onix will evolve into Steelix. When it’s become Steelix, you then need the other player to transfer Steelix to you, and it’s yours. Alternatively, you can always have another player trade you an Onix with Metal Coat, and it’ll evolve into Steelix this way, and you won’t have to make a second trade.