The Stolen Sapphire: A Samantha Mystery (American Girl Mysteries Series)

Samantha and Nellie set sail for Europe in 1906. Also aboard the ship is a world-famous archaeologist and the legendary sapphire he is carrying to a London museum. When his priceless jewel disappears, Samantha realizes that every one of the first-class passengers is a suspect--and one of them must be the thief!