Marvin Redpost Series, Book 8: Magic Crystal?

One afternoon after school, Marvin goes home to Casey’s house. It’s a really cool place that once was a fire station. It even has a pole you can slide down. Casey shows Marvin her special magic crystal. It used to be a normal rock but last night it was hit by lightning so now it can make your wishes come true. But can it really make your wishes come true? Casey and Marvin decide to test the powers of the magic crystal.