sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

Hardness: 3-4 (Mohs scale); Color: Dark grey to charcoal, often with greenish hues from moss/lichen; Luster: Dull or earthy; Structure: Planar layers (fissility), fine-grained texture; Specific gravity: Approx. 2.5-2.8.

Hardness: 3-4 (Mohs scale)
Luster: Dull or earthy

Identified March 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the compaction of silt and clay-sized mineral particles (mud) over millions of years. This specimen appears consistent with Paleozoic or Mesozoic formations common in forested temperate regions like the Appalachian or European highlands.

Uses & applications

Used primarily in the manufacture of cement, brick, and tile. If high in organic content, it can be a source of natural gas or oil. Low-grade slabs are used in landscaping and rustic walkways.

Geological facts

Shale is the most common sedimentary rock on Earth, making up about 70% of the sedimentary rock in the crust. Fossils are most likely to be preserved in this type of fine-grained sedimentary rock due to the low-oxygen environments in which it settles.

Field identification & locations

Identify by its ability to break along flat planes (fissility) and its soft, scratchable surface. It is often found in riverbeds or woodland outcrops. Collectors look for shale with fossil imprints or pyrite inclusions.

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