I can't say if it is a geofact or an artifact. The pics are incredibly blurry. Can you get better pics? What I can say is it looks alot like some of the hematite fossils I have. Hematite would definitely set off your detector, and it should be mildly (or strongly) attracted to a magnet. You could try a streak test, (the lid on the tank of your toilet on the back rough side will work) does it streak red? black? A local university could do a specific gravity test that would go a long way in helping determine what the material is. A geologist could probably also determine if it is a fossil or not. He may be able to tell you if it was hand made or nature made, (Like seeing tool marks under a microscope) I will say that Iron Hematite can make some crazy shapes, both bulbous and angular. I have examples of hematite that are almost perfectly cubic, and some that were cubic, but then worn down by being stream tumbled, making them look very interesting! And, like I said, I have fossils that look very similar to what you have....but we need more info!