sedimentary

Shale

Argillaceous Sedimentary Rock

Mohs hardness: 2.5–3, Color: Gray to olive-green, Luster: Dull/Earthy, Crystal structure: Microcrystalline (clastic), Cleavage: Fissile (splits into thin layers), Specific gravity: 2.4-2.8

Hardness: 2

Identified June 29, 2026More sedimentary →

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Physical properties

Mohs hardness: 2.5–3, Color: Gray to olive-green, Luster: Dull/Earthy, Crystal structure: Microcrystalline (clastic), Cleavage: Fissile (splits into thin layers), Specific gravity: 2.4-2.8

Formation & geological history

Formed from the compaction of silt and clay-size mineral particles (mud) in calm-water environments like lake bottoms, river deltas, and floodplains over millions of years.

Uses & applications

Used as a source material in the production of cement and bricks. It is also an important oil and natural gas reservoir (shale gas) and can be used in the ceramics industry.

Geological facts

Shale is the most common sedimentary rock in the Earth's crust. It often preserves fossils with great detail due to the fine grain size of the original mud.

Field identification & locations

Identified in the field by its characteristic 'fissility'—the ability to break into thin, flat sheets. It is found globally in sedimentary basins. Collectors should look for signs of leaf or fish fossils between layers.