This stone looked nearly the same, but it wasn't. It was the Weakened Stone. And because of its faded color, the stone's magic was much weaker and had trouble granting wishes completely. A wish, for example, to change a girl into a boy (or vice versa) would rarely change them all the way, etc. It just couldn't do a complete job of granting wishes.
Zoe walked towards the spot where she usually met up with her friends before school. And as she approached Athena, she took out the stone, completely unaware that it wasn't the same stone she inherited from her grandfather.