sedimentary

Shale

Clastic sedimentary rock (primarily clay and silt minerals)

Hardness: 3 on Mohs scale; Color: Grey, black, or brown; Luster: Dull/Earthy; Structure: Fissile (breaks into thin flat layers); Cleavage: Perfect basal-like cleavage along bedding planes; Specific Gravity: 2.4-2.8

Hardness: 3 on Mohs scale
Color: Grey, black, or brown
Luster: Dull/Earthy

Identified March 24, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the lithification of mud (clay and silt) in low-energy environments like lake bottoms, river deltas, and deep ocean basins. Ages range from the Archean Eon to the present day.

Uses & applications

Used in the production of cement, bricks, and tiles. It is also a primary source rock for petroleum and natural gas (shale gas) via hydraulic fracturing.

Geological facts

Black shales contain organic matter that can turn into fossil fuels over millions of years. Shale is the most common sedimentary rock in the Earth's crust, making up about 70% of the sedimentary rock record.

Field identification & locations

Identify in the field by its distinct layering and tendency to split into thin sheets. If it smells earthy when wet, it likely contains clay. Found worldwide in basins and road cuts. Collectors should look for fossils, which are commonly preserved in shale layers.

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