sedimentary

Pyrite Concretion

Iron Sulfide (FeS2) in Sedimentary Matrix

AI-generated identification · may be incorrect

Hardness: 6-6.5 (for the pyrite crystals); Color: Brassy yellow crystals on earthy brown matrix; Luster: Metallic (crystals) to dull (matrix); Crystal structure: Isometric (cubic or pyritohedral crystals)

Hardness: 6-6
Color: Brassy yellow crystals on earthy brown matrix
Luster: Metallic (crystals) to dull (matrix)

Identified June 4, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in anaerobic (oxygen-poor) sedimentary environments, often in shales or mudstones, where organic matter decays and reacts with iron and sulfur in the groundwater to precipitate crystals around a nucleus.

Uses & applications

Primarily collected as geological specimens; historically used to create sparks for starting fires and in the production of sulfuric acid.

Geological facts

Pyrite is famously known as 'Fool's Gold' due to its metallic luster and brassy hue. Concretions like this often form in perfect spheres or discs depending on the surrounding pressure during formation.

Field identification & locations

Identify in the field by looking for metallic, cubic crystal growths protruding from a softer, rounded sedimentary rock. Commonly found in coal beds and marine shale deposits.