sedimentary

Shale or Mudstone

Argillaceous sedimentary rock

Hardness: 3 on Mohs scale; Color: Grey to brownish-grey; Luster: Dull/Earthy; Crystal structure: Non-visible fine grains (clay/silt); Cleavage: Fissile (breaks into thin layers in shale) or blocky (in mudstone); Specific gravity: 2.4-2.8

Hardness: 3 on Mohs scale
Color: Grey to brownish-grey
Luster: Dull/Earthy

Identified May 31, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the compaction of silt and clay-sized mineral particles over millions of years. This process, known as lithification, typically occurs in low-energy environments like lake beds, river deltas, or deep ocean floors.

Uses & applications

Primarily used in the production of cement, brick, and tile. It is also an important source rock for natural gas and petroleum (shale gas).

Geological facts

Shale is the most common sedimentary rock on Earth, making up about 70 percent of the sedimentary rocks in the Earth's crust. It often contains well-preserved fossils due to the fine-grained nature of the sediment.

Field identification & locations

Identify by its fine-grained texture (grains are too small to see with the naked eye) and earthy smell when wet. Common in regions with ancient basins or river floodplains. Collectors look for shale with fossil imprints.