sedimentary

Shale

Fissile mudrock (composed primarily of clay minerals and quartz)

Hardness: 3 on Mohs scale (soft); Color: Grey, charcoal, or brownish-grey (due to organic content/iron oxides); Luster: Dull/Earthy; Structure: Fine-grained, clastic; Cleavage: Exhibits fissility (splits into thin, flat layers).

Hardness: 3 on Mohs scale (soft)
Luster: Dull/Earthy

Identified March 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the compaction and lithification of silt and clay-sized mineral particles (mud). It forms in low-energy environments like lake beds, river deltas, or floodplains where fine particles can settle out of suspension.

Uses & applications

Used in the manufacture of brick, tile, and pottery. It is a major component in the production of Portland cement. It also serves as a critical source rock and seal for oil and natural gas reservoirs (shale gas).

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 contains well-preserved fossils because of the fine-grained nature of the sediment.

Field identification & locations

Identify in the field by its tendency to break into thin plates or 'books' (fissility). It will often feel smooth to the touch and can be easily scratched with a steel nail. Commonly found in basin areas and ancient lakebeds worldwide.

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