You really can't compare a Coticule, an Escher or J nat. They're different stones, and impart different edges. We don't really assign grit ratings, because they're naturals, but we guesstimate. The Coti would be the lowest, Escher in the middle, and J nat the highest (at least for the most part)

I was reading through this thread again, and saw the fingernail test. I still use it today, mostly on Coticules, If you can see the scratch, it's a soft stone, If you can't, it's hard. When you have many stones, and have seen different results, this test makes more sense. Now, with Coticules, I have always found softer ones, to be better cutters, and the harder, usually greenish to pale yellow to be better finishers. IME. YMMV.