Kathryn McGuire is always getting in trouble and then complaining about the picture they put in the newspaper. Her father has just bought her a big, expensive diamond, and her boyfriend, Ernest Hilliard, has his eye on it. He is actually the leader of a gang of crooks -- given his character is named 'Rodney', everyone should realize it. He steals it. Fortunately, the manager of her father's ranch is Tom Mix, he's in town for his annual confabulation with his boss, and he recovers it with just a few simple stunts. Then, because Miss McGuire has gotten in trouble again, her father decides to stash her at the ranch, whither Hilliard follows; it's an awfully big diamond, just like it says in the title.Tom Mix's last silent is from FBO, rather than Fox, a lesser studio that knew they had a superstar on their hands, so there's lots of good camerawork, and efficient direction by Eugene Forde. The stunts are not as grand as they had been at his former studio, but they're still very impressive for a man in his late forties who looks ten years younger. Second-billed Tony the Horse gets some nice profile shots in, and the whole thing is pleasingly disposed of in little more than 60 minutes.Tom would take a couple of years off while the movies figured out how to make talkie westerns. He toured with the Sells Floto Circus, then back into the movies with Gower Gulch producers. He was still a big star, but the majors weren't interested in paying him enough. Neither had he saved any of the reputed millions he had made over the years. He died in an auto crash in 1940, aged 60.Not many of Mix's almost 300 films survive; his Fox movies largely perished in the 1937 fire that destroyed most of Fox's silent production. This one held out at the Library of Congress, where silent film preservationist and accompanist Ben Model found it in pristine condition and got it out to the public in 2023 with one of his typically excellent scores.