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The novaculite Arkansases which have a comparable relative density would too feel smooth to touch, but as per the reference they are coarse with an equivalent grit of 600-800.

Lap down a large...

Anthracite differs from ordinary bituminous coal by its greater hardness, its higher relative density of 1.3–1.4g/cm3. As a comparison, a coarse, soft Arkansas is considerably higher than this at...