Here's a quote from Alex at The Japan Blade which amplifies Maxim's comment about how natural stones work. This is the bible for me.
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Natural sharpening stones, the type found near Kyoto but also in other parts of Japan, are complex in their material make up and contain tens if not
hundreds of different elements and compounds. Some of the minerals and fossilized organic material act as cutting and polishing agents while
some make up the binder portion of the stone that holds everything together. The harder minerals like chert, a form of flint, do most of the cutting
while clays mostly make up the binder. Users of Japanese stones notice the contrast between the hard steel and the soft iron after sharpening, in
Japan this is called Kasumi. Kasumi is a word that describes the fuzzy or hazy look objects take on when viewed over hot summer ground. The
kasumi look is desirable to most Japanese tool users but few understand how this effect is achieved.
Taking into account the hardness of these blades, in the Rockwell 60-65 range, only a fraction of the available abrasive grit mix in these stones will
actually polish the hard steel cutting edge, the clay certainly will not. But conversely because the iron backing on laminated blades is so soft almost
every thing in the grit mixture will affect the polish of this softer iron material, even the softer flakier grit particles.
During the sharpening process the soft iron has been honed and reduced in mass by the effects of all of the grinding compounds working in
unison. The chert which cuts the soft iron like butter and the clays, salts, radiolarians and even some silica that is a know element of some of the
older wrought irons will help to sharpen or reduce the jigane soft iron as it is coaxed out of the iron base. The kasumi effect is basically the result of
all of these lesser abrasives working together to sort of massage the surface of the iron, none of the abrasives acting on their own to over power
the other, a little bit like Judo, which translates as "the soft way". The iron is changed and reduced and sharpened, but in a soft way.
The hard tool steel takes on a polished mirror look, some suggest it has the look of chrome. This microscopic scratch pattern in the hard steel that
makes it look polished has been cut by only the hardest of the hard abrasive particles and it looks regular and finished. The soft iron on the other
hand looks dull and complicated. This is the contrast in the polish that is achieved with the natural stones. Synthetic stones give an over all highly
polished bevel while the natural stones allow the soft iron to look soft.