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Gotcha. The distinction between ALL of these stones you mention is density. (They use specific gravity but it's basically the same distinction). There were actually several "grades" of Washita stones...

"Parallel genre" and "similar in composition" seemed to say that to me, perhaps I misinterpreted. Mea Culpa. They are composed of pretty much exactly the same stuff though, not similar.

Nope, Washita is pretty much exactly the same in composition as a Hard, Black or Translucent Ark. 99% - 99.5% or 99.9% silica, forget which one it is. Actually I'm thinking 99.5% now that I've had a...

Washita stones are composed of the same material as the "harder" varieties of Ark but are considerably less dense. This means the abrasive peaks are father apart, which equates to a slightly coarser...

Uhhh. Arks are damn near 100% microcrystalline silica. The closest stone found in nature to 100% silica. Usually something like 99.9% IIRC. They are not quartzite, that is a different animal, though...