Based on the brassy color, it's metallic luster, and it's resistance to breakage, I would say it's pyrite. Try to scratch glass with it (be careful not to break the glass). If it scratches, it is likely pyrite. If a nail can't scratch it, it is likely pyrite. You could test it's streak by rubbing it on a piece of hard unglazed ceramic. If it leaves a black mark (maybe with a hint of green), then it is likely pyrite.
Based on a approximate volume of 458 mL (that of the Monster), which is 458 cm3, and the density of pyrite of 5 gm/cm3, then sample should weigh about 2.3 kg (5 lbs). Galena (see wienein comment below), a less brassy and less hard (easier to scratch) metallic mineral, has a higher density of 7.6 gm/cm3, which would give is a mass of 3.5 kg (7.7 lbs), closer to what you are showing in the picture. Galena has a light grey streak and contains lead.