The Nabob Affair(1960)

Serge, alias Count Fornari, is hired by a mysterious employer to try to steal the Nabob, an exceptionally valuable diamond that Jeannette, alias Aménita (Perrette Pradier), the companion of a billionaire, wears as a locket. She covets the diamond and steals it from the safe. About to be caught in the act, she keeps it hidden in a handkerchief in her hand and manages to conceal it in Serge's pocket during the hotel ball. When she rejoins the billionaire, he asks her to open her hand, but the diamond is gone. The result is a chase full of misunderstandings and twists and turns between the three protagonists.