Shale

Physical Properties

Hardness: 3 on Mohs scale; Color: Gray to dark charcoal; Luster: Dull/Earthy; Structure: Fine-grained, fissile (easily splits into thin layers); Composition: Clay minerals, quartz, and calcite.

Formation & Geological History

Formed through the compaction and lithification of silt and clay-sized mineral particles. Typically deposited in low-energy environments such as river deltas, lakes, or deep ocean floors over millions of years.

Uses & Applications

Primary source for natural gas and oil (oil shale). Used in the manufacturing of cement, brick, and tile. Historically used as a material for writing tablets and walkways.

Geological Facts

Shale is the most common sedimentary rock on Earth, accounting for about 70% of the rocks in the Earth's crust. It is a major source of fossils because it forms in environments where organisms can be buried quickly.

Field Identification & Locations

Identify in the field by its tendency to break into flat, thin plates (fissility). It will not fizz with acid unless it contains significant calcite. Common worldwide in riverbeds and road cuts.

Identified on: 4/24/2026

Mode: Standard