Here's something I was looking for last night from Dan's Whetstone website - their gravity chart.
https://www.danswhetstone.com/inform...ne-grades-101/
"Although there may be only a slight difference in grain size from one grade to another, the effective abrasiveness of each grade is related to the density, or compactness, of the individual, bladed quartz crystals that compose Novaculite. The U.S. government measures the specific gravity of Novaculite and assigns density ranges for the Soft and Hard classifications. Dan’s Whetstone Company, Inc., follows these standards for Soft and Hard stones. We also further define Hard grade Novaculite into the True Hard, Black, and Translucent categories.
Density Standards
Soft Arkansas 2.20-2.30
Hard Arkansas 2.30-2.45
True Hard Arkansas 2.50+
….Colored Translucent 2.50+
….Translucent 2.50+
….Black Arkansas 2.50+"
End quote
Seems to me to make distinctions between the last four grades is speculative and conjecture. Based on my experience with the Translucent and Black Arkansas (what I used to call Hard Black), there is indistinguishable difference.