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I'm assuming by Norton Hard stone he means a vintage True Hard/Surgical black.

No, they're not likely to do a density test on every single rock - and specific gravity is what they're graded by going by Dan's. But, as far as Naturals go Arkansas stones seem to have an uncannily...

When the mine was young, they were likely working from the outside in. And when it comes to Arkansas stones, they're surprisingly uniform across the grade. Translucent and hards are something like...