I dont know. Have seen them found with artifacts. We always said paint pot and in the pocket it went !! May be a geo something but used by the ancients?
They are Concretion Cups and most of them are actually a natural formation, but we finds all kinds of them on the Ft Ancient sites we hunt so they obviously were brought into the site and used for something. I have found a few that have been altered in some way by either pecking, grinding, or drilling. A buddy of mine has one that is cone shaped and was drilled at the top of one edge and was probably used as a Tinkler or a Pendant.