I'll let you know when I work it out
I
think that it has a lot to do with the nature of the abrasive particles and the scratch patterns they leave. Coticules and BBWs contain garnets which break down easily and have obtuse facets: in practice this means that the stones work fairly quickly with new cutting edges being constantly exposed, but the abrasive action is relatively mild and the scratches shallow. That's why a soft coticule like mine forms slurry very easily (that in itself has a slight edge-rounding effect) and gives a very smooth edge - very comfortable to shave with, but it doesn't feel so very sharp. I find it difficult to get good hanging hair test passes from a coticule without stropping - that's got to be technique to some extent, but it's also down to a lack of the micro serrations which catch the hair.
The other extreme is the diamond plate. If you set a bevel on one of those at, say, 600 grit, it will feel very aggressive and "raspy" sharp - it will shave arm hair very easily and may very well pass a HHT - this isn't the kind of edge you'd want to put anywhere near your face though. What you have here is an edge with deep scratches in it, giving it a set of teeth like a saw which are just the thing for grabbing hairs. The diamonds have acute facets, and on account of their hardness don't break down regularly. Scale that edge down 20 times or so and it's something like what you get from a Spyderco. Much smoother to be sure, and certainly shaveable, but it's still jagged and has a lot of "grab". HHTs are a breeze with that stone, as are nicks and slices.
For my purposes the right sort of compromise lies closer to the smooth end of things, so the thuringian works well. The particles are held in a harder matrix and slurry doesn't form spontaneously (you have to rub the hone either with a diamond plate or another chunk of the same stone), so there's less tendency for the edge to round over, but the abrasive action is still quite mild. However, people with tougher beards than mine may want more attack, which is why I've been playing about with some alternatives recently. Hard black Arkansas has been promising (but oh so slow), so has a greenish mystery hone with a glassy hard surface which takes the edge towards the "ninja sharp" (copyright: Mikael) end of the spectrum without it being too unpleasant to use. I'm only guessing, but that stone probably has a lot in common with the Charnley Forest hones.
In summary, smoother: soft matrix, slurry, abrasive particles with obtuse facets; sharper: sharp edged particles, and all other things being equal a hard matrix, no slurry.
Disclaimer: the above may be a load of old cobblers, and there's loads of omissions. I've thought long and hard about this and done a bit of experimentation recently, but it's just my opinion as it stands to date.
