Okay rock hounds, is this ...

[DragonsEye]

iron slag? Was walking along an unpaved track picking up interesting/colorful rocks here and there. Don't have any steel processing plants nor railroads in that immediate vicinity but that doesn't always mean anything. The rocks in Q are quite heavy for their size and hard (ie. don't scratch easily).

 

[Cthulhu138]

Looks like slag to me.

 

[SubRosa]

That's definitely waste from iron smelting, but strictly speaking it isn't slag. Slag is everything that was originally in the ore except most of the iron, and is not dense. That's probably a "crust" of impure iron that formed between the product iron and the actual slag.