I was given what I think is a sharpening stone, it is 9 1/2" x 3" x 1", black or dark grey, and weighs 1.6lbs. There is a depression on one face with the word 'Battersea' below an oval with a capital letter 'A' in the middle of the oval. I was thinking it might be an Arkansas stone but seems terribly light in weight. Can anyone identify this. Thanks for any help.