sedimentary

Shale

Mudstone (composed primarily of clay minerals like kaolinite, illite, and smectite, with quartz and calcite)

Hardness: 3 on Mohs scale (relatively soft/brittle); Color: Gray, black, or brown; Luster: Dull/Earthy; Structure: Clastic, fine-grained; Cleavage: Exhibits fissility (splits into thin layers); Specific Gravity: 2.2-2.8

Hardness: 3 on Mohs scale (relatively soft/brittle)
Color: Gray, black, or brown
Luster: Dull/Earthy

Identified March 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the lithification of mud and silt-sized particles deposited in slow-moving water environments such as lake beds, river deltas, and floodplains. Most deposits range from the Paleozoic to Cenozoic eras.

Uses & applications

Used in the production of cement, bricks, and tile. Fine-grained shales are often the source rock for oil and natural gas (shale gas) via hydraulic fracturing.

Geological facts

Black shales are rich in organic matter and are often the most important source of fossil fuels. Shale is the most abundant sedimentary rock, accounting for about 70% of the sedimentary rocks in the Earth's crust.

Field identification & locations

Identify in the field by its tendency to break along flat, parallel planes (fissility). It will often feel smooth but slightly gritty and can be scratched with a penny. Commonly found in river basins and regions with a history of ancient lakebeds.

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