You will find that many Pokémon have specific evolution requirements. These exact requirements are found in Pokémon Brilliant Diamond and Brilliant Pearl. If you are looking to hunt down specific Pokémon, make sure you understand these requirements. For example, Onix has an evolution requirement if you plan to evolve it into Steelix. Here's what you need to know about how to evolve Onix into Steelix in Pokémon Brilliant Diamond and Brilliant Pearl.
The only way to evolve Onix into Steelix is if you have an Onix wearing Metalcoat. You can get Metal Coat in several ways. One such method is to advance in the game and receive it from Byron on the Iron Islands. However, you can also acquire it by capturing a Magnemite in the Grand Underground. However, capturing a Magnemite with Metalcoat is extremely difficult.
Once you have the Metal Coat, the next step is to trade Onix with another player you trust. During the trade, Onix will evolve into Steelix. When it becomes Steelix, you need the other player to transfer Steelix to you, and it's yours. Alternatively, you can always ask another player to swap an Onix for Metal Coat, and that way it will evolve into Steelix and you won't have to make a second trade.