Gummi Stone Gray is more abrasive than our Red or Tan Gummi Stones.  Gray Gummi Stone size is 3" X 1" X 3/4".  Use the gray stone for initial prepping of the edge.  Also works for as a final step in the sequence of sharpening with a diamond file then polishing with a stone, follow those two steps up with the Grey Gummi to detune tips and tails.