Pyrite, iron sulfide (FeS2), is a common mineral in many rocks. Geochemically speaking, pyrite is the most important sulfur-containing mineral. (more below)
Pyrite occurs in this specimen in relatively large grains associated with quartz and milky-blue feldspar. Pyrite has a Mohs hardness
of 6, a brass-yellow color and a greenish black streak
Pyrite resembles gold slightly, but gold is much heavier and much softer, and it never shows the broken faces that you see in these grains. Only a fool would mistake it for gold, which is why pyrite is also known as fool's gold. Still, it's pretty, it's an important geochemical indicator, and in some places pyrite really does include silver and gold as a contaminant.
Pyrite "dollars" with a radiating habit are often found for sale at rock shows. They are nodules of pyrite crystals that grew between layers of shale or coal.
Pyrite also readily forms crystals, either cubic or the 12-sided forms called pyritohedrons. And blocky pyrite crystals are commonly found in slate and phyllite.
Other Diagenetic Minerals
Other Hydrothermal Vein Minerals