This is a chuck of nickel ore from one of the greatest nickel deposits of the world. Mindat states that the deposit is "a giant polymetallic deposit hosted within one of the Earth's largest impact craters (diameter 250 km). Initially discovered in 1856, but the main development started in 1883.
Polydymite is silvery when fresh with alteratins to Annabergite. It has a density of 4.8 and a hardness of 4.5 to 5.5.