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Indeed, a single stone can give a wide range of results. I'm still learning but beyond pressure I'd add condition of the stone surface, slurry vs plain water, water vs glycerine or oil depending on...

I think you want something like this:
JapaneseNaturalStones.com: The Big Test

but I don't think natural stones are as simple as just giving them a grit rating... I'm sure someone who knows more...