Pyrite

Pyrite, iron sulfide (FeS₂), is a common mineral in many rocks. Here pyrite occurs in relatively large grains associated with quartz and milky-blue feldspar. Pyrite is the most important sulfur-containing mineral. This page shows several different guises of the mineral. Pyrite has a Mohs hardness of 6, a brass-yellow color and a greenish black streak.

Pyrite resembles gold slightly, but gold is heavier and 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" are often found for sale at rock shows. The image below is about twice life size. This object is a nodule 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. Both of these are shown below. And at the bottom is some blocky pyrite crystals set in a paving stone of slate.

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