sedimentary

Shale

Fissile Mudstone (primarily composed of clay minerals and quartz)

Hardness: 3 on Mohs scale; Color: Gray, brown, or rusty due to iron oxides; Luster: Dull/Earthy; Structure: Clastic, fine-grained; Cleavage: Perfect fissility (breaks into thin, flat layers); Specific Gravity: 2.4 - 2.8.

Hardness: 3 on Mohs scale
Color: Gray, brown, or rusty due to iron oxides
Luster: Dull/Earthy

Identified May 10, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed from the compaction of silt and clay-sized mineral particles, commonly called 'mud'. It forms in low-energy environments like lake bottoms, river deltas, and floodplains. Most accessible shale dates from the Paleozoic to Cenozoic eras.

Uses & applications

Used in the production of cement, bricks, and tiles. Specially processed 'oil shale' is a source of natural gas and petroleum. Crushed shale is used as a base material for roads.

Geological facts

Shale is the most common sedimentary rock on Earth, accounting for roughly 70 percent of the rocks in the crust. It often contains well-preserved fossils because the fine grains seal out oxygen.

Field identification & locations

Identify in the field by its 'fissility'—the tendency to split into thin, brittle sheets. It is commonly found in basins or near coal seams. Collectors should look for impressions of leaves or marine life between the layers.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary

Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary