Not me, you fool! - I mean the broken-in-half Roman rotary-quern.
Such a quern works as described in this video. When complete, this stone would have had a hole in the top, and a hole in the side (you can see two-thirds of it at the point of breaking) which would have had a horizontal stubby stick inserted.
Wheat, barley or rye would have been dribbled down the hole while turning this upper-part of the quern on a flat stone base, probably incised with lines to collect and channel the resulting flour. The flour came out all around the edges and was collected onto something like a leather hide underneath the quern, a flat board, or a larger flat stone.
It was a very slow method of grinding, taking (I believe) more than an hour of hard work to grind enough grain for a loaf.
This object we inherited from an ancient relative, who lived in West Northumberland in the vicinity of many Roman Ruins - which is why we believe it is Roman.
But to be honest, I can't identify it with any degree of confidence. You might be able to see that it is fluted with vertical hollows - and I haven't been able to find any other similar-looking quern online in order to try and get a better date.
Anyway, it's very old, and an impressive chunk of masonry.