Sherlock Holmes: Terror by Night (1946)

In London, Lady Margaret Carstairs and her son, Roland, engage renowned detective Sherlock Holmes to accompany them back home to Scotland as protection for their priceless gem, the Star of Rhodesia. When Roland turns up murdered and the Star disappears, Holmes and his friend Dr. Watson investigate, only to discover another murder, and a strange connection with Watson's acquaintance Maj. Duncan-Bleek.