sedimentary
Chert (Iron-stained)
Chert (Microcrystalline and Cryptocrystalline Quartz, SiO2)
Hardness: 7 (Mohs scale), Color: Brown to tan with reddish-orange iron oxide staining, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown to tan with reddish-orange iron oxide staining, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation in ground water. This specimen appears to have undergone significant weathering and oxidation of iron impurities.
Uses & applications
Historically used for stone tools and arrowheads. Currently used as an abrasive, in road construction (aggregate), and occasionally for lapidary work if highly patterned.
Geological facts
Chert is so hard it can produce sparks when struck against steel, which made it vital for prehistoric fire-making. It is one of the most chemically stable and durable rocks in Earth's crust.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel blade) and its sharp, shell-like (conchoidal) fractures. Common in limestone formations. Look for smooth, waxy surfaces on fresh breaks.
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