Shale

Physical Properties

Hardness: 3 on Mohs scale; Color: Grey, brown, or tan; Luster: Dull/Earthy; Structure: Laminated and fissile (splits into thin layers); Cleavage: None (cleaves along bedding planes); Grain size: Very fine clay and silt-sized particles.

Formation & Geological History

Formed through the compaction and lithification of silt and clay-sized mineral particles, typically in quiet, low-energy environments like lake beds or deep ocean basins. Geological age varies significantly from billions of years to recent deposits.

Uses & Applications

Primary source for brick and tile manufacturing; used in the cement industry; significant as a host rock for natural gas and petroleum (shale gas).

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 often contains well-preserved fossils because of its fine grain size.

Field Identification & Locations

Identify by its 'fissility'—the ability to break into thin, flat plates. It has a smooth, muddy feel when wet. Found in basins and along riverbanks. Collectors look for shale because it is the most likely rock to contain impressions of leaves or small prehistoric life.

Identified on: 4/27/2026

Mode: Standard