The Earth Plate is one of the held items you can give a Pokémon. If you are a fan of ground types, this item is for you since it boosts the power of ground-type moves. This item isn’t attainable until later in the game. Here is where you can find the Earth Plate in Pokémon Brilliant Diamond and Shining Pearl.
Before you can get the Earth Plate, you first need to obtain the hidden moves Strength, Rock Smash, and Surf. You also need to be able to use them outside of battle. To use these moves outside of battle, you first need to defeat the gym leaders in Oreburgh City, Pastoria City, and Canalave City. Once you are able to use these moves outside of battle, head over to Oreburgh Gate (the cave next to Oreburgh City).
Take the north path in Oreburgh Gate to reach a set of stairs that go down to a room below. Head south in the room until you find a body of water. Use surf and head east to the left edge of the water. Here you will see a veteran trainer blocking a staircase. Defeat the trainer and head down the stairs. There are two items here. The item to the south is the Earth Plate.