This narrow but thick and long Nakayama tomae stone would be used in a specialized craft in Japan for sharpening narrow chisels for wood carving, mortise & tenon joinery projects.
It is 195x45x38mm, and weighs 766 grams. 
The back of the stone can also be utilized with its surface of 35mm wide. The kawa skin on the back that sparkles along with the ocher kawa skin suggests it is from the Nakayama mine.