Iron pyrite, or 'fool's gold' naturally forms cubic crystals when the conditions are right. They come away from their soft matric very easily. The perfect examples are picked out by the collectors and sold individually for considerably more, so each of these crystals will have a small imperfection, but they are still beautiful objects.
They are from Navajun, in La Rioja, Spain.
Roughly 2.5cm across the face.