An Everstone is a unique item. You’ll want to place it on a Pokémon that you want to prevent from evolving. You can also prevent a Pokémon from evolving by spamming the B button while it’s evolving into its next form, but this can become tedious. The Everstone does this automatically. Here’s what you need to do to find an Everstone in Pokémon Brilliant Diamond and Shining Pearl.
The best way to find an Everstone is to explore the Grand Underground. You can reach this location once you receive the Explorer’s Kit from the old man in Eterna City, the one who lives next to the Pokémon Center. While in the Grand Underground, you’ll need to dig at the orange sites on the map. These are searching sites where you can find rare items, such as the Everstone.
You can increase your chances of finding items like the Everstone by collecting the Diglett and Dugtrio that appear on the ground. You’ll see the bonus steadily rising up on the left side of your screen, underneath your Grand Underground map. The higher this is, the more chance you have of finding rare items. When you reach 40, you’ll have increased chances of finding items, shiny Statues, and Shiny Pokémon in the area for four minutes.