sedimentary

Limonite-coated Shale

Ferruginous Siliciclastic Shale

AI-generated identification · may be incorrect

Hardness: 3-5 (shale) with 4-5.5 (limonite coating); Color: Dark grey to black with reddish-brown to mustard-yellow oxidation; Luster: Dull to earthy; Structure: Fissile, platy cleavage.

Hardness: 3-5 (shale) with 4-5
Luster: Dull to earthy

Identified July 17, 2026More sedimentary →

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Physical properties

Hardness: 3-5 (shale) with 4-5.5 (limonite coating); Color: Dark grey to black with reddish-brown to mustard-yellow oxidation; Luster: Dull to earthy; Structure: Fissile, platy cleavage.

Formation & geological history

Formed through the lithification of clay and silt particles in low-energy environments like lake beds or deep ocean basins. The reddish-brown coating is Limonite, formed by the oxidation of iron-bearing minerals (like pyrite) within the rock.

Uses & applications

Shale is used in the production of bricks, cement, and as a source of natural gas (shale gas). Specimens with distinct iron staining are prized by amateur collectors for their color contrast.

Geological facts

Shale is the most common sedimentary rock on Earth, making up about 70 percent of the rocks in the Earth's crust. The 'rust' seen here is actually a mixture of hydrated iron oxides.

Field identification & locations

Identified by its ability to split into thin flat pieces (fissility) and the characteristic earthy smell when wet. Found in sedimentary basins worldwide. Look for flat, crumbly outcrops in road cuts.