sedimentary
Siltstone Pebble
Siltstone (composed of microscopic quartz and clay minerals)
Hardness: 3-5 (Mohs scale), Color: Brownish-red to grey, Luster: Dull/Earthy, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal or irregular fracture), Specific Gravity: 2.5-2.8
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Physical properties
Hardness: 3-5 (Mohs scale), Color: Brownish-red to grey, Luster: Dull/Earthy, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal or irregular fracture), Specific Gravity: 2.5-2.8
Formation & geological history
Formed through 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 reddish hue suggests oxidation of iron-rich minerals.
Uses & applications
Primarily used for construction aggregate, decorative landscaping, and occasionally as a base material in pathing. Siltstone is rarely used in jewelry due to its relative softness.
Geological facts
Siltstones are distinct from shales because they lack fissility (the ability to split into thin layers). They often preserve trace fossils like ripples or animal tracks due to the fine grain size.
Field identification & locations
Identify in the field by its smooth, gritty feel (finer than sandstone but coarser than clay) and lack of layering; commonly found in riverbeds and glacial deposits. Collector tip: look for specimens with interesting mineral veins or fossil impressions.
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