Three Courts, or Murder During the Ball

A monstrous crime has been committed in a Russian provincial town. During a ball at the estate of a local wealthy man, a girl, the daughter of the house's owner, is murdered. Upon examination of the body, it is discovered that a diamond ornament belonging to the deceased is missing. Later, the diamonds are found, and it turns out that their theft was not the motive for the murder at all. So what was it?