C Is for Crystal — K.T. Tomb

When a business associate of Thyri's father hires the Alpha Adventurers to locate couriers who disappeared while transporting three antique Swarovski vases, they travel to Luxembourg to investigate. Thyri, Travis, Adam, and Savannah take on the case of a long, bitter rivalry between families who have a history of stealing the vases from each other. Travis and Adam go to France to investigate, while Thyri and Savannah sleuth the clues in Luxembourg City.