From the Super Mario Wiki
The Crystal Berry is a key item found in Paper Mario.
Posie is connected at the roots to the Crystal Tree and she shakes the tree to make a Crystal Berry fall out. According to Rosie, the Crystal Berry is more beautiful than the Water Stone, and she will eagerly give the stone to Mario for the berry. Mario must then return the Water Stone to Lily so she can refill her pond before she dries up, which enables Mario to access the Bubble Berry tree.
Unlike the other berries Mario finds in Flower Fields, there is only one Crystal Berry, and Rosie will not return it once Mario gives it to her.
The Crystal Tree and Posie's area is at the end of the southwestern path of Flower Fields.
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