Based on appearance and density 3000-4000 kg/m3, it might be:
Iron sulfide (pyrite and/or marcasite) concretion. It might a have a sweet sulfide smell.
Manganese oxide nodule - This would be soft and should be able to scratch with a knife blade.
Density is a bit high for basalt, but could be iron rich.
- Igneous Gabbro/pyroxenite cobble - Density matches.
You may need to break or cut in half with a rock saw to know more.