I purchased this from a wonderful dealer in New Hampshire whose cabinet of curiosities includes objects decommissioned from museums and science departments as well as gathered from estate collections all over.  She didn't know the origins of this stone and neither do I, but it sure looks like a throwing stone to me! If not certainly used as a tool of some sort, with a couple of divets on the bottom--one just the right shape for a thumb gripping it. There is a number written on one side in what looks like pencil and looks to be 6999, and a fragment of a paper label as well. Whatever the case, quite a satisfying thing to hold and wonder about, about the size of a baseball.

Approx 3" in diameter, sits flat, perfectly sized to fit into the palm of one's hand with fingers wrapped around it.