Shale

Physical Properties

Hardness: 3 on Mohs scale; Color: Black, dark gray, or charcoal; Luster: Dull or earthy; Crystal structure: Microcrystalline/clastic; Cleavage: Fissile (breaks into thin layers); Specific gravity: 2.4 - 2.8

Formation & Geological History

Formed from the compaction and lithification of silt and clay-sized mineral particles, typically in low-energy environments like lake beds or deep ocean basins. Ages range from Precambrian to Cenozoic.

Uses & Applications

Used in the production of bricks, tile, and Portland cement. It is also an important source of natural gas and oil when it contains organic matter (oil shale).

Geological Facts

Shale is the most common sedimentary rock on Earth. It often contains well-preserved fossils because the fine-grained sediment protects small organisms from decay.

Field Identification & Locations

Identify in the field by its tendency to split into thin, flat sheets (fissility). It feels smooth but slightly gritty and will not fizz with acid unless it contains calcite.

Identified on: 5/20/2026

Mode: Standard