The Hard Arkansas stone offers the next step up in sharpening with natural Arkansas stones. It will refine the edge and begin to polish your blade edge. Hard Arkansas stones are graded fine and while the actual hardness of the stone may vary due to it being a natural stone it will offer a range similar to a man made stone in the range of 800-1000.
I have discontinued using these natural stones for the ViperSharp kit. The natural stones can’t seem to be made to the exact measurements needed. Since the stone thickness is vital for a professional sharpening system to function 100% these stones will be available only as an add-on and you should keep in mind that a digital angle finder or similar tool would be highly recommended to assure the same angle as the other man made stones.