Because it's not oval enough. Get a dictionary.
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Happiny is the only Pokemon that can evolve from a Oval Stone in Pokemon Diamond, Pearl, and Platinum. You will need to max its Happiness during the morning or day, let it hold an Oval Stone, and then level it up once. It will, then, evolve into a Chansey.
If held, it evoles Happiny into Chansey when leveled in the day.
It evolves to chansey by holding a oval stone and making it happy. First, it hatches from it's egg and obviously is a Happiny and when it's leveled up holding an Oval Stone, it evolves into Chansey. Chansey evolves into Blissey via happiness. So the evolution is Happiny, Chansey, and Blissey.
Any time you feel like it, but make sure to give it to her as a hold item.
they evovle certain pokemon. Oval stone is used to evolve Happiny into Chansey. shiny stone evovles Togetic into Togekiss and Roselia into Roserade.