sedimentary

Shale or Siltstone

Sedimentary Clastic Rock (Variable mineral composition including quartz and clay minerals)

Hardness: 3-4 on Mohs scale; Color: Gray to dark charcoal; Luster: Dull/Earthy; Crystal structure: Non-crystalline/Microcrystalline; Cleavage: Fissile (breaks into thin layers); Specific gravity: 2.4 - 2.8

Hardness: 3-4 on Mohs scale
Color: Gray to dark charcoal
Luster: Dull/Earthy

Identified March 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the lithification of mud, silt, and clay particles deposited in low-energy environments like lake beds, river floodplains, or deep ocean floors. Can vary in age from millions to hundreds of millions of years old.

Uses & applications

Used as a raw material in the manufacture of cement, brick, and tile. Historically significant as a source of fossil fuels (oil shale) or used as a filler in various industrial products.

Geological facts

Shale is the most common sedimentary rock on Earth, accounting for about 70 percent of the rocks in the Earth's crust. It often contains well-preserved fossils because of the fine grain size that settles around organic matter.

Field identification & locations

Identify in the field by its tendency to split into flat, thin plates (fissility) and its smooth, fine-grained texture. Commonly found in basins or areas with historical water activity. Collectors should look for impressions of leaves or marine life on the flat surfaces.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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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