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Question is how does one tell their stone is 20k grit if they have never tried a 20k synthetic stone? Or 50-60k grit?
For naturals grit is not so important, it is more important how the stones work...

you can't determine grit for naturals as they are not made from spherical particles, except coticule AFAIK.
Jnats and Eschers are made from flaky particles so assigning grit for those has no...

what size are those stones, 5x2?