This is iron pyrite, also known as "Fool's Gold." While it is not nearly as valuable as gold, it is a very useful mineral. What is it used for?
Pyrite is used in the manufacture of sulfur dioxide and sulfuric acid, although sulfur from other sources is rapidly replacing it in these processes. In the past, it was used for starting fires, and instead of flint in wheel-lock guns. In the heyday of crystal radios, it was used as the "detector" in the radio. It is sometimes even cut into jewelry, but currently, the most common use of pyrite is as a gold ore. That's right! Pyrite and gold often form in the same environment, and it is not unusual for pyrite to contain enough gold to make it worth while mining. Before you start hoarding pyrite, be aware that gold ore is valued by the ton, not the ounce as with gold. You are going to need a LOT of pyrite to strike it rich.