Originally a pyrite crystal, with the outer portion now replaced by iron oxides and the core still unaltered pyrite? 
Sounds like Alfredo nailed it. Replacement of pyrite by goethite or other iron oxides is certainly known in pegmatites. I've collected such pseudomorphs from pegmatites in southern California as well as in Maine. I've seen them on pegmatite specimens from Pakistan or Afghanistan. Although the "pyrite core" doesn't fit, is uraninite a possibility? No, it's goethite ps pyrite. Very common. Hardly magnetite. You can check with a compass to see if it deviates.
