Seconding Mike, I was going to say Washita Arkansas stone but when I looked it up, Washita's are white. Maybe the pores of the stone have metal in the pores giving that mottled look. I learned how to sharpen on a stone like you have pictured back in 1987( I was told it was a Washita). This was the first rough stone. Afterwards a translucent grey Arkansas stone was used to polish the edge.