Difficult to give a definite answer without seeing it first-hand, and without knowing where it came from. IF it really is that heavy, I suspect it may be a piece of mine slag - waste left over from smelting metal ores. Slag usually looks a bit more ripply, however. If it is not slag, it could be a rock rich in iron ore (there are several sorts).
You could try bringing it close to a compass, and seeing it it affects it. If so, that at least shows it has significant magnetic-mineral (probably iron) content.
It is probably magnetite, haematite, or slag. (Note: slag may or may not be affected by a magnet.)
I suggest you do a streak test: magnetite will be grey; haematite will be brown; slag will probably not give a streak.
If you're willing to break it open, give it a go. Wear protective specs if you start wielding hammer and chisel. You might find crystals of black metal-like magnetite 'inside'. I can't say whether it might be finely or coarsely crystalline, perhaps both.
For a detailed list of properties of magnetite, see http://www.webmineral.com/data/Magnetite.shtml.
With a bit of inventiveness, you should be able to devise ways to test some of those properties. Doing so would help you prove or disprove the 'magnetite' hypothesis.
You've already done streak - your dark grey is close enough to their black.
If in doubt, density and hardness would be worth a go.
Of general interest, that web page explains the origin of the name 'magnetite':
'Named for Magnes, a Geek shepherd, who discovered the mineral on Mt. Ida, He noted that the nails of his shoe and the iron ferrule of his staff clung to a rock.'
i put a compass on the floor and pounded on the floor so see if movement on the floor would make the compass move and it didnt so i slolwy scooted the rock the compass and the compass start slowly spinning! what could this mean??
im still confused if its worth anything. its rusting the streak test passed ( did it on the underneath of my toilet lid where the it wasnt glossed) i hung a magnet from my cabinet and it swung over to my rock. the compass test passed. i cracked it open a little and it has sparkles in it, its heavy and has cracks which i think could be called impact cracks. what do you think about all this?