One of the most common arguments for using diamond sharpening plates is that they are flat and do not get dished during their working life. The first point is only partly true, as the flatness of the plates depends a great deal on the quality of the plate and the manufacturing method. But it is true that they do not become hollow during use, unlike other kinds of sharpening stone. http://www.taideasharp.com/knife-sharpener/diamond-knife-sharpener/