What is "fool's gold" made of?

The nickname "fool's gold" refers to the mineral pyrite because of its similarity in shape and color to actual gold. Pyrite differs from gold in its crystalline structure and shares more chemical similarities with iron sulfides and marcasite than with gold.

Pyrite is a sulfide mineral frequently found in conjunction with other sulfides. It is also sometimes found alongside gold, arsenic, nickel, silver and cobalt. Unlike gold, pyrite is susceptible to oxidation and rusting, altering both its color and its structure, making it unsuitable for construction materials. Decaying pyrite can leak sulfuric acid and cause environmental damage. Pyrite releases heat that can cause explosions in mines.