sedimentary
Shale or Mudstone River Pebble
Argillaceous sedimentary rock
Hardness: 3-4 (Mohs); Color: Grey, black, or brown; Luster: Dull/Earthy; Crystal structure: None visible (clastic); Cleavage: Fissile or non-cleavable; Density: Low to moderate.
- Hardness
- 3-4 (Mohs)
- Color
- Grey, black, or brown
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 (Mohs); Color: Grey, black, or brown; Luster: Dull/Earthy; Crystal structure: None visible (clastic); Cleavage: Fissile or non-cleavable; Density: Low to moderate.
Formation & geological history
Formed from the compaction of silt and clay-sized mineral particles. The rounded shape indicates significant mechanical weathering and transport in an aqueous environment like a river or beach.
Uses & applications
Primarily used in the production of cement, bricks, and occasionally as structural fill in construction; also studied for fossil content or oil potential.
Geological facts
Shale is the most common sedimentary rock on Earth, making up approximately 70 percent of the rocks in the Earth's crust. It often preserves fossils because of the fine-grained nature of the sediment.
Field identification & locations
Identify by its fine-grained texture, dark color, and rounded edges. Common in riverbeds and coastal regions. It may feel slightly smoother or waxier than sandstone.
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