What you have found is a piece of coal. Probably anthracite or "hard coal".
Being from the "coal region" of PA, I find the damn stuff all the time.
One of my favorite spots is a farm field near a railroad [which was laid in the 1850's.] and had a house MANY years ago. I get 40-42 on the AT Max all the time and so far every time , in that field, it has been ANOTHER lump of coal.
Of course that same field has given me a 1925 SLQ,a 1775 KGIII, 1908d dime, and some other good stuff.
It's 105 acres of great hunting and has also given me 3 or 4 very nice arrow heads.