The Stolen Sapphire: A Samantha Mystery (American Girl Mysteries Series) by Sarah Masters Buckey, Jean-Paul Tibbles
Samantha and Nellie set sail for Europe aboard an ocean liner in 1906. Also traveling on the ship is a world-famous archaeologist with the legendary sapphire he discovered. Before the archaeologist can deliver his treasure to a London museum, the priceless jewel disappears. Samantha realizes that every one of the ship's first-class passengers is a suspect-and one of them must be the thief! Can Samantha and Nellie uncover the real culprit and find the missing jewel? The book includes a "Looking Back" essay and a glossary of French words used in the story.