A Naif is the outer surface of an unpolished and uncut diamond. When cutting a diamond a small bit of this outer surface is sometimes left on the surface, usually at the girdle. This unpolished bit is known as a natural and is indicative of the cutter conserving as much of the rough diamond as possible.
1,109-Carat Diamond, Lesedi La Rona, from the Lucara Mine, Botswana.
Photo Courtesy of the New York Times.