The Moonstone Castle mystery

When Nancy Drew receives a valuable moonstone as a gift from an unknown person, she is amazed and puzzled. But it is only the first of several startling events in this complex mystery that challenge the ingenuity of the pretty sleuth. Why are the Bowens--a missionary couple who recently returned to the United States--having so much trouble finding their missing seventeen-year-old granddaughter?
Nancy gets involved in mystery when she receives a valuable moonstone from an unknown person.