Those who like to use defensive moves will want to grab the Light Clay as they traverse the world of Sinnoh. This object increases the amount of time it moves as Bright Screen and Reflection last. Obtaining this item is no easy task, however, as it requires multiple hidden moves and cannot be recovered until quite late in the game. This is where you can find this item in Pokémon Brilliant Diamond and Brilliant Pearl.
Quite late in the game, the story will direct you to Lake Verity, Lake Valor, and Lake Acuity. After checking out the first two lakes and facing Team Galactic, you will need to climb Mount Coronet. Travel to Celestic Town to start your journey. Head east out of the city to find the cave entrance.
Once inside the cave, use the movement force hidden on the boulder to the north. After getting him out of the way, he continues north through the cave until you come across a set of stairs leading down. Go down the stairs and you will enter a large room full of fog. Use Defog if you got it from the NPC in the Great Marsh to clear the area.
Now that the room is clear, go north and go down the first flight of stairs you see on your right. Use Surf to swim through the water to a small landing with an object on top. Collect the item to retrieve the light clay.