Pyrite

Hardness: 6-6.5 (Mohs scale), Color: Golden yellow, brassy yellow (often with an iridescent tarnish), Luster: Metallic, Crystal Structure: Isometric (typically cubic or pyritohedral crystals, but often found as massive, granular, or stalactitic forms), Cleavage: Poor/indistinct, Specific Gravity: 4.9-5.2

Pyrite forms in a wide variety of geological environments, both igneous, metamorphic, and sedimentary. It often forms in hydrothermal veins, associated with other sulfide minerals. It can also precipitate from anoxic (oxygen-deficient) waters in sedimentary environments, such as black shales and coal beds. It typically forms at moderate to high temperatures. Its geological age varies widely depending on the specific formation event.

Historically, pyrite was used as a source of sulfur for the production of sulfuric acid, which is vital for many industrial processes. It was also used in early radios as a detector crystal. Today, its primary industrial use is still as a source of sulfur, and it can also be a minor ore of iron. In jewelry, polished pyrite is sometimes used as a decorative stone, especially in its more attractive crystal forms. It is also a popular collector's mineral due to its appealing crystal shapes and metallic luster.

Pyrite is commonly known as "Fool's Gold" due to its striking resemblance to gold, especially to the untrained eye. This nickname arose during various gold rushes when prospectors were often fooled by its golden luster. Despite its nickname, pyrite is not worthless; it's an important source of sulfur and iron. It can also oxidize in the presence of water and oxygen to form sulfuric acid and iron oxides, which can cause environmental problems like acid mine drainage.

To identify pyrite in the field and distinguish it from gold: Gold is much softer (hardness 2.5-3 Mohs) and can be scratched with a knife or even a fingernail; pyrite is harder. Gold leaves a yellow streak on an unglazed ceramic plate (streak test), while pyrite leaves a greenish-black streak. Gold is much denser and heavier than pyrite. Pyrite often forms distinct cubic or pyritohedral crystals, which gold rarely does. Pyrite is commonly found worldwide in various rock types, including sedimentary rocks (like shales), metamorphic rocks, and hydrothermal ore deposits. Good specimens are found in Spain (Navajún), Peru, Italy, and various locations in the USA. For collectors, looking for well-formed crystals or large masses is key.