This is bornite (Cu5FeS4), also known as peacock copper, for obvious reasons. It's from Montana, courtesy of our neighbour, and beautiful, and heavy.
As you can see, it's small, but if I threw that at your forehead with sufficient force, you wouldn't get up again. A pretty way to die.
Why, yes! I am back in Hild world, thank you for asking. I'm researching and pondering Book II and so thinking violently again. Lovely!
As I ponder I'll be using this lump of shiny goodness as a paperweight for all my notes and vaguely hoping for an opportunity to use it for it's true purpose...