The SP-650 Stone Grader allows you to adjust the sharpening characteristics of your Original Grindstone or Blackstone Silicon. These grindstones have adjustable sharpening characteristics equivalent to between 220 and 1000 grit. The fine side of the stone grader provides finer sharpening characteristics, while the coarse side is used to maintain the grindstone and reactivate the grit for more effective and coarser sharpening.