sedimentary

Shale

Lutaceous sedimentary rock

Hardness: 3 (Mohs scale); Color: Dark grey to charcoal black; Luster: Dull or earthy; Crystal structure: Non-crystalline/fine-grained; Cleavage: Fissile (readily splits into thin layers); Specific gravity: 2.2 - 2.8

Hardness: 3 (Mohs scale)
Color: Dark grey to charcoal black
Luster: Dull or earthy

Identified March 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the compaction and lithification of silt and clay-sized mineral particles (mud). It originates in low-energy environments such as river deltas, lake bottoms, and deep ocean basins over millions of years.

Uses & applications

Used primarily in the production of cement, bricks, and tiles. It is also a significant source rock for natural gas and petroleum through hydraulic fracturing.

Geological facts

Shale is the most common sedimentary rock on Earth, making up approximately 70% of the sedimentary rock in the Earth's crust. It often preserves fossils because of the fine-grained nature of the sediment.

Field identification & locations

Identify in the field by its distinct layering and dull appearance; it will often break into flat plates or thin chips when struck. Common in Appalachian mountains and river basins. Collectors should look for fossil imprints (ferns, shells) between layers.