sedimentary

Shale

Shale (clay-rich mudstone)

Hardness: 3 on Mohs scale; Color: Gray, black, or reddish-brown; Luster: Dull/Earthy; Structure: Fissile (breaks into thin layers); Cleavage: Perfect basal-like cleavage along bedding planes; Specific Gravity: 2.2 to 2.8

Hardness: 3 on Mohs scale
Color: Gray, black, or reddish-brown
Luster: Dull/Earthy

Identified June 27, 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. It typically forms in slow-moving water environments like lake bottoms, river deltas, and floodplains.

Uses & applications

Used in the manufacture of bricks, tiles, pottery, and as a component of Portland cement. It is also an important source for natural gas and petroleum (oil shale).

Geological facts

Shale is the most common sedimentary rock on Earth, accounting for about 70 percent of the rocks in the crust. Many shales contain significant fossils due to the fine-grained nature of the sediment that preserves detail.

Field identification & locations

Identify by its thin, plate-like layers and the fact it can often be scratched with a fingernail or penny. It is found globally in sedimentary basins. Collectors look for shale layers containing plant or marine fossils.