sedimentary

Limonite Concretion

Limonite (FeO(OH)·nH2O) Pseudomorph

AI-generated identification · may be incorrect

Hardness: 4-5.5 (Mohs), Color: Dark brown to blackish-brown, Luster: Dull or earthy, Structure: Amorphous or cuboidal (if replacing pyrite), Opaque, Specific Gravity: 2.7-4.3

Hardness: 4-5

Identified June 19, 2026More sedimentary →

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Physical properties

Hardness: 4-5.5 (Mohs), Color: Dark brown to blackish-brown, Luster: Dull or earthy, Structure: Amorphous or cuboidal (if replacing pyrite), Opaque, Specific Gravity: 2.7-4.3

Formation & geological history

Formed via the oxidation of iron-rich mineral precursors (like pyrite) in sedimentary rock environments where water and oxygen are present to chemically weather the original mineral.

Uses & applications

Primarily of interest to mineral collectors and as educational specimens; historically used as a low-grade iron ore.

Geological facts

These are often called 'Propeller Stones' or 'Devil's Dice' when they retain the cubic shape of the original pyrite they replaced. A concretion forms by the precipitation of mineral cement between particles.

Field identification & locations

Identify by its heavy weight relative to size, dark brown streak, and often unusual geometric or rounded shapes found in clay or shale beds. Common in many sedimentary regions worldwide.