Pyrite

Pyrite is an iron sulfide mineral (FeS₂), the most abundant of the sulfide minerals and long nicknamed fool's gold for its brassy, metallic luster. Pyrite forms across a remarkably wide range of geological environments - from boiling hydrothermal vents on the ocean floor to cold, oxygen-poor sedimentary basins. This versatility makes it the most common sulfide mineral on Earth.

In hydrothermal systems, pyrite crystallizes from sulfur-rich fluids as they cool below about 500 degrees Celsius. The crystals from Navajun, Spain - some of the most perfect cubes found anywhere in nature - grew in marl (a type of mudstone) from low-temperature fluids over millions of years, which allowed the slow, orderly atomic arrangement that produces those eerily geometric shapes.

Pyrite also forms in sedimentary environments when iron reacts with hydrogen sulfide produced by bacteria in oxygen-poor muds. This is how the tiny pyrite framboids (raspberry-shaped clusters of microscopic crystals) found in black shales originate. In fossils, pyrite often replaces the original organic material atom by atom, preserving intricate details of ancient organisms in gleaming metallic gold.

Pyrite's metallic luster and brass-yellow color make it one of the most recognizable minerals. The classic confusion is with gold, hence 'fool's gold.' But the distinction is straightforward: gold is soft (2.5 Mohs), malleable, and has a gold streak. Pyrite is hard (6.5 Mohs), brittle, and leaves a greenish-black streak. Scratch it on unglazed porcelain and the question is immediately settled.

Pyrite's crystal habits are also diagnostic. Perfect cubes, pyritohedrons (twelve-faced forms), and octahedrons are all common and distinctive. Its high specific gravity (5.0) gives it a satisfying heft. On broken surfaces, pyrite shows conchoidal to uneven fracture rather than cleavage.