A: "Not ballast, they used regular round stone for that. Based on the area you found it, petobego pond area was a large Marl clay quarry for the Elk Rapids Portland cement co. My best educated guess is it was a processed and fired piece of marl clay as a quality test. Hence the markings for a batch number. I am on board of that region’s underwater preserve council and trained underwater archeologist….. suggest you take to Elk Rapids historical museum for confirmation. Very nice and knowledgeable folks there."