The Oval Stone is an item in the Pokemon games which can be used to evolve certain Pokemon. For example, it can evolve Chansey.
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The Pokemon that can use oval stone in Pokemon platinum is Happiny. It is the pre evolve form of chansey. Mostly every chansy you catch have a oval stone.
You can evolve happiny with an oval stone after its friendship is high
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The Oval Stone evolves a Happiny into Chansey however it needs to be leveling up while it's holding the Oval Stone during the day time.
I think you evolve Happiny into Chansey with the oval stone? They have to hold it and level up I think?