Amethyst — Rebecca Lisle

Amy jumps at the chance to leave her gloomy aunt and uncle and go to the mountains, and live with the King of the Rock People. But once there, she finds that she must go to stay with the alien clan, the Wood People, and betray them by kidnapping Copper's wolf cub from them. Worse still, when she meets the Woods she likes them, and her traitorous position there makes her more and more uncomfortable. A mysterious stranger turns up at the house, and kidnaps their beloved wolf cub before Amy gets the chance. Amy is soon engaged in the chase through the winter landscape to a final showdown with her King.