The Mazarin Stone

S7 E5: Sherlock Holmes is in the Highlands and Mycroft is called in to recover the Mazarin stone - a diamond due to be returned to France and which has been stolen by Count Sylvius from the Whitehall Museum. Watson investigates an American trying to remove Nathan Garrideb from his room with the promise of a vast inheritance. It transpires the cellar below is equipped for cutting diamonds. Will Sylvius get away with it and what of the diamond?