Although we are not experts in stone identification we believe this to be a black Arkansas sharpening stone.

Dark grey colour, reasonably flat and smooth top surface with a few light scratches, please see images.

This stone measures 7¹/₁₆ inches long by just under 2¹/₂" wide by 1¹/₄" deep (180 x 63 x 31mm).

Clear cutting/dressing marks to the four sides.

This stone weighs 2lb 1³/₈oz or 994g.

The last image shows water on the surface.

Will benefit from flattening/dressing before use.

Please see all images.