The Weirdstone of Brisingamen

Neither Susan nor her brother, Colin, ever thought that war would be waged over a simple gemstone in her bracelet. But that's what happens when the children visit Alderley Edge, a spooky place in a remote part of England. There, they meet the wizard Cadellin, who needs the stone to rouse his allies in the never-ending battle between good and evil. But when the stone vanishes, Susan and Colin must find it before the forces of evil use it to destroy all the goodness that ever existed in the world.