I don’t think I’ve ever seen a rock with so many natural fractures, I would have picked it up too. But I don’t see any percussion ripples and some of the fractures have a near 45 angle. The divot on top looks like a pot lid fracture from fire, but the rock doesn’t look like it’s been heated. I would have to say natural from this one pic, but you’re definitely looking at the right things.
It appears natural. The rust color looks like oxide of some sort. But since I'm no expert I suggest showing it to the staff at a natural history museum.