sedimentary

Shale

Argillaceous Rock

Mohs hardness: 2-3 (soft), Color: dark gray to brown, Luster: dull/earthy, Structure: laminated/fissile clastic, Cleavage: perfect parallel to bedding planes

Identified May 14, 2026More sedimentary →

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Physical properties

Mohs hardness: 2-3 (soft), Color: dark gray to brown, Luster: dull/earthy, Structure: laminated/fissile clastic, Cleavage: perfect parallel to bedding planes

Formation & geological history

Formed from the compaction of silt and clay-sized mineral particles, commonly in low-energy environments like lake beds or deep ocean basins. Ages range from the Archean to the Holocene.

Uses & applications

Used in the manufacture of cement, brick, tile, and as a filler in paint. It is also a primary source rock for oil and natural gas (shale gas).

Geological facts

Shale is the most common sedimentary rock on Earth, accounting for roughly 70 percent of the rocks in the crust. It often contains well-preserved fossils because of its fine grain size.

Field identification & locations

Identify in the field by its ability to split into thin sheets (fissility). It will often have a smooth feel but will not scratch glass. It is commonly found in river deltas and floodplains.