sedimentary

Shale

Mudstone/Shale

Hardness: 3 on Mohs scale; Color: Gray, green, or brown; Luster: Dull/Earthy; Structure: Clastic, split-able into thin layers (fissility); Cleavage: Perfect along bedding planes; Specific Gravity: 2.4-2.8

Hardness: 3 on Mohs scale
Color: Gray, green, or brown
Luster: Dull/Earthy

Identified May 10, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the compaction of silt and clay-size mineral particles (mud). It typically forms in low-energy environments like lake bottoms, river deltas, and floodplains where fine sediments can settle.

Uses & applications

Used as a source of oil and natural gas (oil shale), as a component in cement manufacturing, and in the production of bricks and tiles. Sometimes used as a filler in road construction.

Geological facts

Shale is the most common sedimentary rock on Earth, making up approximately 70% of the rocks in the Earth's crust. It often contains well-preserved fossils because it forms in quiet water environments.

Field identification & locations

Identify in the field by its distinct layering and tendency to break into thin, flat fragments. It will not fizz with acid (unlike limestone) and is soft enough to be scratched by a copper penny.