I had found this on the Yorkshire coastline in 2013, lying loose on the beach. This is a rectangular cuboid structure. Each end is relatively smooth. Three of the larger faces have a textured surface but the middle photograph is taken of the smooth end face which is slightly concave. The other which is an underside (since the specimen stands easily when rested on this side) is smoother but with some evidence of the same texturing but of a lower density than the other three surfaces.
Does anyone have an idea of what this is?