Antique natural sharpening stone in wooden box

Description

An antique natural sharpening stone in a wooden box.  It is dished out in the middle and has oil crud on it.  It may have chips, scratches, stains, dents, dings and other conditions associated with old, used natural material.  Looks to my eyes like there are layers in the stone running lengthwise but would need a cleaning know for sure.  Please see the pictures for the size measurements and condition.  Thanks for looking!