sedimentary

Shale

Argillite / Shale

Hardness: 3 on Mohs scale; Color: Gray to bluish-gray; Luster: Dull/Earthy; Crystal structure: Microcrystalline (clay minerals); Cleavage: Perfect basal cleavage (fissility); Specific gravity: 2.2-2.8

Hardness: 3 on Mohs scale
Color: Gray to bluish-gray
Luster: Dull/Earthy

Identified July 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the compaction of silt and clay-size mineral particles. These sediments are deposited in low-energy environments such as river deltas, lakes, or deep ocean basins over millions of years. This specimen likely dates from the Paleozoic or Mesozoic era depending on location.

Uses & applications

Used primarily in the production of cement, crushed stone for road construction, and as a source rock for oil and natural gas (shale gas). Higher quality specimens are used for pottery or brick making.

Geological facts

Shale is the most common sedimentary rock on Earth, making up approximately 70% of the sedimentary rock in the Earth's crust. It is often the host rock for fossils due to its fine-grained nature.

Field identification & locations

Identify in the field by its fine-grained texture and its tendency to break into thin, flat layers (fissility). It will not fizz with acid unless it contains calcite. Common in sedimentary basins worldwide.