How is pyrite formed?

Pyrite forms due to a chemical process initially caused by the decomposition of organic matter in bacterial sulfate reduction. The bacterial sulfate reduction produces an inorganic anion known as bisulfide, which reacts with iron minerals to form iron sulfides, the most common of which is pyrite.

Pyrite is often known as "Fools Gold" because it has a similar color and shape to gold. It is also known as iron pyrite or iron disulfide, as it forms due to complex chemical reactions between decaying organic material and iron minerals.

As it is formed due to a chemical process and decaying matter, pyrite is unlike real gold in that it gives off a foul sulfurous smell often likened to rotten eggs.