I believe, after seeing the Pokemon 4ever movie, it may have something to do with Celebi??
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A King's Rock can be found in Celestic Town. Go to the left (or left side of birdhouse thing) and use your Dowsing Machin.
in the celestic tow cave in the center of the town
Hearthome is before it and after that is Celestic town.
She is by the cave
She is in Celestic town. Fight the Galactic Grunt protecting the ruins in Celestic Town and Cynthia's grandmother will appear.