Lots of old tools are mysterious. Like any piece of modern tooling, found discarded, can be baffling too I guess. My Dad was loading gravel at the quarry years

Lots of old tools are mysterious. Like any piece of modern tooling, found discarded, can be baffling too I guess. My Dad was loading gravel at the quarry years ago, the site of the local Meskwaki campground in the 1800's. Found a rock sitting there with a cross-hatch pattern worked into the surface, about half an inch deep! Six or eight inches across. A stone the size of a cantaloupe. Each groove seemed to be rubbed in the stone (a very hard stone) and overlapping. That is, one groove would cut across another, as if they'd been rubbed in serially and not all at once. Each about a finger-width. Almost like somebody had taken their thumb and rubbed the lines in the surface, though that is impossible, it was far too hard. Kept it in his office on a shelf for years, just a conversation piece. Never figured it out.