Here is the rest of the lapping story. While tired and still making a mess on the back porch, I thought, "well, I might as well try the 400 diamond plate and see what happens." I had previously discounted this idea, because i didn't want to unnecessarily wear it out on this hard stone. But I was ready to make some different muscles sore, so I went to work. It was amazing, dark green slurry poured off of that thing and I was done in maybe 15 minutes and able to progress on to finer sand paper.

I took a carbon steel razor that already had an edge that I liked, and gave it 20 laps at a time until the new scratch patter had replaced the previous polish (around 150 total light pressure laps maybe). It's appearance was somewhat harsh but consistent, maybe visually comparable to a 4k-5k pattern. I was skeptical that it was fine enough for a shave. After untreated linen and leather, I was getting very good HHT results. The next morning's shave was 8 out of 10 keen and about 4-10 comfortable; overall not very pleasant, but in the name of science I completed my normal routine without swapping to a different razor.

Overall impression is that this is just an odd stone. I believe to be a natural stone (do any old synthetics show cutting wheels marks on every side?). I will probably give it a go as a pre-finisher, but that isn't really a process I would normally use. In any case, I acquired it from my dad who thinks he might have gotten it from his father, so it will certainly remain a very important part of my humble collection.

-Mickey