Put a few days of research into this. Sorry, I'm new to geology. Going to read a book on it when I have time, I've got one lying in my basement somewhere.
It seems to be a layer of what looks like sort of granite rock, but a solid color. Then a calcite inside, which I don't think matches up. Took me about 10m each of throwing it against another rock (found these at the beach, didn't have access to tools.)
On Moh's scale, it's less than 2.5 (fingernail), I'm not sure how to test for less.
Found on Long Island, NY.
Ask me any follow up questions if needed.