Having been woodworking now for well over 30 years, I have a collection of various stones that I have tried and retired. But nothing has convinced me of late to give up on Japanese water stones when sharpening planes and chisels. They are clean to use, relatively cheap and, critically, are very easy to return to flat when they wear.
When honing a cutting edge on a steel blade, you are progressively over three or four stages reducing the size of the scratches on the two faces which meet up to make the sharp point. The much vaunted “mirror” edge simply refers to the stage where we cannot easily see the scratches with the naked eye and hence it looks smooth and shiny.
This process therefore requires several sharpening stones with finer and finer surfaces (325 grit, then 1200 grit, then 6000 grit). Sometimes these follow on from the use of an extremely coarse (often around 60 grit) grindstone. At the Centre students like the modern water-cooled sandstone grinders now available.