When you're dealing with a natural stone, the properties of a waterstone, the grit is measurable and it's different. When you get to noviculite (Arkanas stone), it gets tricky. They're rated by density, which in turn determines the size of the pieces in the slurry.Whether coticule or Jnat there is no real "grit" rating. I rather rate them by their hardness; the harder the faster and in essence higher grit.
I think we can work on what certain stones are considered harder than others and maybe different methods to hone with them.
Somebody actually did the research and used lapping film at various grits to compare the size of the scratches from the stones, and various grits.
Arkansas Sharpening Stone Grits
Are Arkansas Sharpening Stone grits finer than we thought? A careful study of Arkansas stone grits by microscopic evaluation. Some surprising results.
www.bestsharpeningstones.com