sedimentary
Siltstone or Fine-grained Sandstone pebble
Siltstone (primarily SiO2 with clay minerals)
Hardness: 6-7 (Mohs); Color: Tan to dull brownish-green; Luster: Dull/Earthy; Crystal structure: Clastic (non-visible crystals); Cleavage: None; Specific gravity: 2.3-2.6
- Hardness
- 6-7 (Mohs)
- Color
- Tan to dull brownish-green
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Tan to dull brownish-green; Luster: Dull/Earthy; Crystal structure: Clastic (non-visible crystals); Cleavage: None; Specific gravity: 2.3-2.6
Formation & geological history
Formed via the accumulation and lithification of silt-sized particles (smaller than sand, larger than clay) in low-energy environments like river deltas, floodplains, or lake beds. These typically date from many geological eras depending on location.
Uses & applications
Largely used as fill material in construction or as river rock for landscaping. Generally lacks industrial or gemstone value unless it possesses unique banding or fossil inclusions.
Geological facts
Siltstones are often differentiated from shales by their lack of fissility (the ability to split into thin layers). Over millions of years, heat and pressure can transform these into metamorphic rocks like slate or phyllite.
Field identification & locations
Identify in the field by its gritty texture (finer than sandpaper but rougher than clay) and dull appearance. It is commonly found in riverbanks, glacial tills, and coastal areas. This specific specimen appears to be a water-worn, rounded river pebble.
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