I am not a geologist but limestone is a sedimentary rock so it was at the bottom of a body of water at some stage. My guess is that it is an igneous rock (looks like granite?) that has been weathered/smoothed by water or wind… Or you could be very lucky and it is a fossilised egg?
Perhaps get in contact with the USGS or Smthsonian - If they are anything like their equivalent bodies in Australia they actually enjoy fielding these sorts of questions from the public.
I friend of my Brother is a geologist who has spent most of his life gold prospecting in Western Australia.
There is an old gold mine that shut down in 1962 not far from where we live and I have found a few flecks when panning in our river. I asked him what to do if I ever found a nugget and he said “tell no one but give me a call and I will tell you what you need to do to lodge a claim”.