The Oval Stone is an item shaped like an egg. It only has one purpose: to evolve Happiny. There’s only one of it in most games, though it can be retrieved by Pokémon like Pachirisu who has the Pickup ability. That said, its location in Sinnoh isn’t that hard to find!
Where to find an Oval Stone in Brilliant Diamond and Shining Pearl
You can get an Oval Stone from the Lost Tower, which is located south of Solaceon Town. It’s in a Pokéball on the second floor in front of the Young Boy.
Your next Oval Stones can be stumbled upon by pure luck. Look for Happinys and Chanseys along Route 209 and 210 and battle them with a Pokémon who has the Pickup ability.
Aside from an Oval Stone, your Happiny needs to be at max friendship to evolve. To know if your Happiny is ready to evolve, open your Friendship Checker and long press on Happiny’s icon. The Pokémon needs to have two big hearts before it's ready. After that, make it hold the Oval Stone, and it will evolve on the next level up.
For more help with Pokémon Brilliant Diamond and Shining Pearl, we at Pro Game Guides have you covered.