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I think Neil is spot-on here.
This is why I try to avoid relating the performance of a natural stone to a grit rating. I see the need for a comparison from one stone to another - but my mind is too...

The Annual Report of the Geological Survey of Arkansas for 1890 Vol III

You can find it in Google books; there are other volumes, but that's one I saved. Don't know how accurate the data is...

It certainly does look similar to mine but I also suppose it could be another sedimentary stone with abrasive qualities.
Grit - the silica xls (abrasive particles) in the Hindostan were, at one...