Pyrrhotite is an iron sulfide mineral related to iron pyrite, which is one of the minerals known as Fool's Gold. Pyrrhotite, although not one of the Fool's Gold minerals, is sometimes called magnetic pyrite as it is weakly magnetic. It is most valued as a collectors mineral.
Sample provided by Carlin Green, USGS. Sample originated from Eagle Mine, Michigan, and is 4.5cm in size.