Shale

Physical Properties

Hardness: 3 on Mohs scale; Color: Gray, greenish-gray, or brownish-tan due to iron oxidation; Luster: Dull or earthy; Crystal Structure: Microcrystalline (clay minerals); Cleavage: Fissile (splits easily into thin plates); Specific Gravity: 2.4 - 2.8

Formation & Geological History

Formed from the compaction of silt and clay-sized mineral particles. It originates in low-energy environments like lake bottoms, river deltas, and floodplains. Typically found in stratigraphic layers ranging from the Paleozoic era to the present.

Uses & Applications

Used primarily in the production of bricks, tile, and pottery. It is also a key ingredient in Portland cement and can be a host rock for natural gas and oil (oil shale).

Geological Facts

Shale is the most common sedimentary rock on Earth, accounting for about 70 percent of the rocks in the crust's upper layer. It often contains well-preserved fossils because of its fine-grained nature.

Field Identification & Locations

Identify in the field by its characteristic 'fissility'—look for thin, flat layers that peel or break away easily. It often feels smooth to the touch compared to sandstone. Found globally in sedimentary basins.

Identified on: 4/27/2026

Mode: Standard