A magic crystal?

By Louis Sachar

Subjects: Humorous stories, Wishes in fiction, Wishes -- Fiction., Friendship, fiction, Magic -- Fiction., Wishes, Friendship in fiction, Magic in fiction, Marvin Redpost (Fictitious character), Magic, Magic, fiction, Fiction, Children's fiction, Friendship -- Fiction., Juvenile fiction, Friendship

Description: When Marvin Redpost's new friend Casey shares with him her magic crystal that can make wishes come true, things get out of control.