Natural sharpening stones, the type found near Kyoto but also in the 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 material like chart, a form of flint, do most of the cutting, while clays mostly make up 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.
Traditional Japanese laminated tool blades were designed with the expectation that they would be sharpened with natural Japanese sharpening stones. This all began in the Edo period when Japan was essentially closed to foreign trade, no sharpening stones were imported from Europe or the U.S. so by default all sharpening stone were domestic. Currently in Japan you will find that active and recently retired blacksmiths are and have been following an unbroken line of traditional forging techniques that are similar if not exactly like the Edo period blacksmiths, and that many strive to achieve the same result of their ancestors. These current blacksmiths are still forging with the expectation that they will have a final polish derived by using Japanese natural tennen toishi.
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 steel, the clay certainly will not. But conversely because the iron is so soft almost every thing in the grit mixture will affect the polish of this soft material.
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 help to sharpen or reduce the jigane soft iron. The kasumi effect is basically the result of all of these abrasives working together to massage the surface of the iron, none of the
abrasives acting 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 is look polished has been cut by 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.
At this time I can only speculate what materials the major brands of synthetic stones are composed them of. The formulas are proprietary knowledge. I would venture to guess however that a stone advertised as a ceramic stone is basically composed of just that, ceramic material & some fancy glue or a fused binder. No one is bragging about Rare Earth Elements or titanium.
As the soft iron bevel is reduced the kasumi effect is created by the myriad of other compounds found in the stone including the clays, salts, radiolarians and even some silica that is a know element of some of the older wrought irons.
In fact visually the honing of the soft steel has been overshadowed by the mass.