I posted the link to the auction this stone came from in the "Did I just buy an Escher?" thread a few days ago, and today I received it in the mail. In that thread, it was unanimously declared to be something other than an Escher, and I think there were 4 or 5 votes for it being a synthetic hone, most likely an India Oil Stone.
Everyone was correct - it is most definitely not an Escher. But it is a natural stone with a couple greenish inclusions that have metallic (pyrite?) flakes in 'em. It is much harder than a C12k (my normal finisher) and took an hour and a half to lap from nearly flat on 400 grit wet/dry paper. I only lapped it up to 600 grit, but will go back this week and finish through 1200.
I tried it out on a quality blade from Solingen that I had finished yesterday on the C12k that was quite keen off the Chinese hone. Forty strokes on water on this mystery stone noticeably improved the edge from the C12k, and I was able to silently pop just the tips off arm hair, even before stropping. In those 40 strokes there was some good feedback, but it didn't release any slurry or visibly remove any metal. (slurry from lapping was light grey) I shaved yesterday with this blade off the C12k and it was good, today I shaved with it after visiting this stone and it was better- I didn't even notice the transition from WTG to ATG under my jawline.
So what could it be? My guess is it's a UK stone. The box it came in looked 100+ years old, it had been used before but there wasn't any oil on it, and gray slurry usually goes with slate. Here's pics: