Pyrite

Pyrite, often called 'Fool's Gold' because of its metallic luster and brassy-yellow hue, is the most common sulfide mineral. Its name comes from the Greek 'pyr', meaning fire, because it emits sparks when struck with iron or flint. Despite its nickname, pyrite is a fascinating mineral that forms perfect cubes and is often found in association with real gold and other valuable minerals.

Formation & Geology

Pyrite forms in a wide variety of ways: in high and low-temperature hydrothermal veins, by segregation from magmas, and in sedimentary rocks. It is often found as replacement minerals in fossils (pyritized fossils). Because it forms at many temperatures and in many environments, it is found all over the world. It is the most common of the sulfide minerals and is a major source of sulfur.

Physical Characteristics

Pyrite is famous for its crystal habit. It commonly forms perfect, sharp-edged cubes with striated faces. It can also form 'pyritohedrons' (twelve-sided faces). It has a hardness of 6 to 6.5, which is much harder than real gold (which is only 2.5-3). It is brittle and has a greenish-black streak, whereas gold has a golden-yellow streak. When exposed to the elements, it can oxidize into iron oxides like limonite.

Industrial & Historical Use

Historically, pyrite was used to ignite gunpowder in wheel-lock guns. Today, its primary industrial use is as a source of sulfur and in the production of sulfuric acid. While it contains a high percentage of iron, it is not usually mined for iron because sulfur is difficult to remove. In jewelry, it is often sold as 'Marcasite' (which is technically a different mineral with the same composition). It is also a favorite specimen for mineral collectors due to its brilliant metallic luster and geometric shapes.

Pyrite in History & Culture

Long before it was known as "Fool's Gold," pyrite was valued for its ability to create fire. The name comes from the Greek "pyr" (fire). Ancient civilizations used nodules of pyrite to strike sparks against flint or steel to kindle fires. In the Victorian era, it was polished into small rose-cut gems and set into silver jewelry, known as "Marcasite jewelry" (though it was actually pyrite, not the mineral marcasite, which is too unstable).