sedimentary
Chert (Flint nodules)
Chert (SiO2)
Hardness: 7 (Mohs scale), Color: Brown, gray, or tan with white weathering rinds, Luster: Waxy to dull, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (has conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown, gray, or tan with white weathering rinds, Luster: Waxy to dull, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (has conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed by the accumulation of microcrystalline silica in marine environments, often replacing original limestone or chalk through a chemical precipitation process over millions of years.
Uses & applications
Historically used for making sharp tools and arrowheads; currently used as construction aggregate, road stone, and occasionally for lapidary work or fire-starting (flint).
Geological facts
Chert is so hard it can scratch steel and glass. It was one of the most vital resources for prehistoric humans due to its ability to fracture into extremely sharp edges.
Field identification & locations
Identify in the field by its waxy luster, extreme hardness, and smooth, shell-like (conchoidal) breaks. Often found as nodules within limestone or chalk layers in cliffs and stream beds.
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