Thank you very much Hershel!
At first I thought this might be Chalcopyrite crystals, but something about the crystal structure didn't seem to fit. To me this looks like two cubes have grown together, but they are not completely cubic. From what I read Bornite sometimes (although rarely) forms nearly-cubical crystals. Pyrite is hardly an option here because of the irredescent colouring of the crystals.
Anyway, I appreciate your answer and will try to do a streak/hardness test on them.