It's not slate or thury/Escher type. When I first took it to the DMT 325 it was very hard like a novaculite stone, it slurrys very thick like paste. Although after getting it on the lapping machine I found out it is not as hard a a translucent Arkansas stone. I was able to lap this in 5 minutes compared to an Ark'y that would have been 2 hours.

But it is still much harder than slate.