Desc: The Big Diamond Robbery: Directed by Eugene Forde. With Tom Mix, Tony the Horse, Kathryn McGuire, Frank Beal. Tom Markham is the foreman of an Arizona dude ranch. He travels to the "big city" for a meeting with the ranch's owner, George Brooks, after which he will accompany Brooks' daughter Ellen back to the ranch. Brooks tells Tom about the Regent diamond, a valuable stone that Brooks has set in a ring for Ellen. Ellen's friend Rodney Stevens hears about it. Stevens, who is actually the boss of a bandit gang, has his men steal the diamond, but Tom manages to get it back. Stevens doesn't give up, though, and hatches a plan that will get him back the diamond and frame Tom for stealing it.