Shale (or Mudstone)

Physical Properties

Hardness: 2-3 on Mohs scale; Color: Dark grey to charcoal black; Luster: Dull or earthy; Crystal Structure: Very fine-grained (clastic); Cleavage: Fissile (splits into thin layers); Specific Gravity: 2.4-2.8

Formation & Geological History

Formed through the compaction of silt and clay-size mineral particles commonly known as mud. This typically occurs in low-energy environments like lake bottoms, river deltas, and floodplains over millions of years.

Uses & Applications

Primarily used in the construction industry for making bricks, tiles, and pottery. In energy sectors, black shale is a significant source of natural gas and oil (oil shale).

Geological Facts

Black shale gets its color from high organic carbon content and often indicates an environment that was low in oxygen (anoxic) during deposition. It is one of the most common sedimentary rocks on Earth.

Field Identification & Locations

Identify in the field by its very fine texture and its tendency to break into thin, flat plates (fissility). It can often contain delicate plant or animal fossils. Found globally in basins and sedimentary outcroppings.

Identified on: 5/2/2026

Mode: Standard