Contractors laying down drainage beside our house (which is in northern New Jersey and was probably built in the 1940s) dug up hundreds of white spheres, each approximately 3/4 inch diameter.
They are of hard material and not easy to break, though we did split one with a sledge hammer. They seem to be stone, all the way through.
Folks at the Bergen County Historical Society were kind enough to give me an opinion, based on a photo and description, that these are white marbles. What do you think, Ray Haupt?