Pyrite (FeS2) is the most common sulfur-bearing mineral in the Earth’s crust and can be found in all major types of rock: igneous, metamorphic, and sedimentary. It is the shiny, brass-colored mineral that you may know as fool’s gold because it looks like gold and has fooled people throughout recorded history (Fig. 1). In some areas, it can be observed at the sides of roads or trails, especially where the ground has been recently disturbed. If you have ever caught the scent of rotten eggs while digging at the beach, in mud flats, or swamps, you were probably in an area where pyrite was forming. That smell is caused by hydrogen sulfide (H2S), and the black or blue muds that you were digging in would have contained microscopic pyrite that formed when the hydrogen sulfide that you smelled bonded with iron in the pore waters (Rickard 2012).