sedimentary

Siltstone / Slate

Siltstone / Low-grade Slate

Hardness: 3-5 on Mohs scale; Color: Dull grayish-brown to dark gray; Luster: Dull to earthy; Texture: Fine-grained, smooth, with subtle layering or flaking along bedding planes.

Hardness: 3-5 on Mohs scale
Color: Dull grayish-brown to dark gray
Luster: Dull to earthy

Identified July 26, 2026More sedimentary →

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Physical properties

Hardness: 3-5 on Mohs scale; Color: Dull grayish-brown to dark gray; Luster: Dull to earthy; Texture: Fine-grained, smooth, with subtle layering or flaking along bedding planes.

Formation & geological history

Formed from the compaction and lithification of silt-sized mineral particles (primarily quartz and feldspar with clay minerals) in low-energy aquatic environments like lake beds or river floodplains. With low-grade metamorphism, siltstone transitions into slate.

Uses & applications

Commonly used as construction aggregate, paving stones, landscaping rock, and occasionally in rough stone tool knapping or decorative crafts.

Geological facts

Fine-grained sedimentary rocks like siltstone and shale make up over 60% of all sedimentary rocks exposed on the Earth's surface.

Field identification & locations

Can be identified in the field by its fine-grained, smooth texture that feels grittier than clay/mudstone but finer than sandstone. Commonly found in riverbeds, ancient lake basins, and shale/slate quarry regions.