"if sarsen stone is so hard that slight wear would not erode in 3000 years than it is too hard to wear from wooden rollers in the first place."
Wear and erosion are different things. Sarsen is not immune to wear (see the polisher thread above) and 1,000 years of use would have left marks. Surprisingly a softer substance can wear a harder as it can pick up and retain grit and other abrasive substances. Mill stones, even the best from the Marne Valley in France need to be dressed and they don't touch one another.
i am aware how two circular stone mill wheels turn against each other. first of all these mills vibrate terribly and are often crude compared to modern metal mill shafts and wheels.... here's what happens. the top wheel must be lower onto the face of the bottom wheel to control the grind. the more you lower it the finer the flour.
sometimes in fine grinding, the mill wheels do bump together a bit from vibration and wear each other away. it is not the flour wearing the stone but the second mill stone or particles from it or inclusions like dirt in the corn. if two mill stones are used together and kept seperated ( as in course grinding) there is never much wear......the wheels only wear a lot when fine close grinding is done.
remember that these wheels weigh a ton and are often supported on primative shafts and bearings. when they are set to grind fine flour they may be only the thickness of a sheet of paper apart.... any vibration and they scrape together...........also heat and expansion can cause them to touch..
the miller's job is to keep them from touching while at the same time getting the finest closest grind on the flour. of course wheels wear out under these conditions.
the proof is this, take a big stone mill wheel and spin it for a very long time in just grain with no other stone meeting or matching it and there is no wear to speak of.