sedimentary
Siltstone / Fine-grained Graywacke
Argillaceous siltstone
Hardness: 3-5 (Mohs scale), Color: Dull gray to greenish-gray, Luster: Dull/Earthy with tiny sparkling mica specks, Texture: Fine-grained to clastic, Cleavage: Poorly developed (not fissile)
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Physical properties
Hardness: 3-5 (Mohs scale), Color: Dull gray to greenish-gray, Luster: Dull/Earthy with tiny sparkling mica specks, Texture: Fine-grained to clastic, Cleavage: Poorly developed (not fissile)
Formation & geological history
Formed from the accumulation and lithification of silt-sized particles (larger than clay, smaller than sand) in low-to-medium energy environments like river floodplains or marine basins.
Uses & applications
Commonly used as fill material, landscaping stone, or in the manufacturing of cement if the chemical composition is suitable. Small tumbled pieces are common beach or river pebbles.
Geological facts
Siltstones are midway between sandstones and shales. The tiny sparkling specks visible in the photo are likely small flakes of muscovite mica that reflect light.
Field identification & locations
Identified by its gritty feel on the teeth (grittier than clay/slate), fine-grained texture, and lack of distinct layering compared to shale. Found globally in sedimentary basins.
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