sedimentary

Siltstone with Weathering Rind

Siltstone (composed of microscopic quartz and clay minerals)

AI-generated identification · may be incorrect

Hardness: 3-4 on Mohs scale; Color: Dark grey exterior with tan/yellowish weathered core; Luster: Dull/Earthy; Crystal Structure: Clastic (fine-grained); Cleavage: None

Hardness: 3-4 on Mohs scale
Color: Dark grey exterior with tan/yellowish weathered core
Luster: Dull/Earthy

Identified May 3, 2026More sedimentary →

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Physical properties

Hardness: 3-4 on Mohs scale; Color: Dark grey exterior with tan/yellowish weathered core; Luster: Dull/Earthy; Crystal Structure: Clastic (fine-grained); Cleavage: None

Formation & geological history

Formed from the accumulation and lithification of silt-sized particles (larger than clay but smaller than sand) in low-energy environments like river floodplains or lake beds. The tan core represents a weathering rind where minerals have oxidized.

Uses & applications

Commonly used as fill material in construction, as a soil conditioner when crushed, or as flagstone if it splits into flat plates.

Geological facts

Siltstone is intermediate between sandstone and shale. When it is extremely fine-grained and fissile, it transitions into shale; when more massive and coarse, it becomes sandstone.

Field identification & locations

Identify in the field by its gritty 'toothy' feel on the teeth (unlike smooth clay/shale) and lack of visible individual sand grains. Often found in layered outcrop sequences.