Hi at first glance it looks like obsidian if it is quite light in weight then that is probably what it is
Big stones?! First I also thought probably not Obsidian but petrofied cole but one stone leads me to believe it could be very dark petrofied wood. So difficult from a photo when you new at this but I am very curious now what it is! It would also be nice for you if it turns out to be graphite, not? 
It’s very heavy there is 230 lbs in 19 pieces the biggest piece weighing 45 lbs! There are little tiny spots that shimmer the rainbow. It is magnetic, It ranges in color from gloss black to dull green and through microscope appears to be olivine spots . Streak test is white it will scratch glass image|375x500
Almost looks like a slag from a smelter plant? I have something that looks similar from a old copper smelting plant that came from melting the ore and removed as a byproduct??
It appears to be obsidian, perhaps from the welded base of a pyroclastic flow or ignimbrite. I once constructed a tunnel through the base of such a pyroclastic flow in Kenya, and the material in the photographs looks nearly identical. The conchoidal fractures typical of obsidian are very clear in the photographs.
But I have also seen similar glassy slag from old brick kilns.