sedimentary
Stream-Rounded Shale or Mudstone
Argillaceous sedimentary rock
Hardness: 2-3 (Mohs scale), Color: light grey to tan, Luster: dull/earthy, Crystal structure: microcrystalline (not visible), Cleavage: splits along bedding planes (fissility), Specific Gravity: 2.4-2.8
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Physical properties
Hardness: 2-3 (Mohs scale), Color: light grey to tan, Luster: dull/earthy, Crystal structure: microcrystalline (not visible), Cleavage: splits along bedding planes (fissility), Specific Gravity: 2.4-2.8
Formation & geological history
Formed from the compaction of silt and clay-sized mineral particles. The rounded shape indicates significant transport in water, likely a river or stream bed, through the process of abrasion over thousands of years.
Uses & applications
Used in the production of cement, bricks, and occasionally as a source for natural gas (shale gas). Smooth specimens are often used for decorative garden paths or meditation stones.
Geological facts
Shale is the most common sedimentary rock on Earth, making up about 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 smooth, water-worn texture and fine-grained, layered appearance. Found globally in riverbeds, deltas, and floodplains. Collectors look for pieces with unique banding/layering.
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