Interesting looking stone, but I can't tell what it is. What grit wet sandpaper did you use? I would try a really coarse 220 grit on a flat surface, if you haven't already, until you get a DMT. The edges of your stone have been beveled. I tried to do that with one of my black Arkansas stones, but it was way too hard. Maybe yours is softer than a black Arkansas. What I hope you have is a large Thuringian. Did any slurry form when you tried lapping on sandpaper? If so, what color was it? Let's see what some of the HAD folks with lots of different stones think, when they wake up.