sedimentary
Chert (often colloquially called Flint)
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 (Mohs scale), Color: Brown, tan, and gray mottled, Luster: Waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown, tan, and gray mottled, Luster: Waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed from the precipitation of silica-rich fluids in sedimentary environments; often replaces carbonate minerals or results from the accumulation of silica-rich microfossils like diatoms or radiolarians.
Uses & applications
Historically used for stone tools and arrowheads; currently used in construction aggregate, road base, and as decorative landscaping stones.
Geological facts
Chert is so hard it can strike sparks from steel, which led to its historically significant use in flintlock firearms. It was the primary material for the Stone Age tool industry due to its sharp conchoidal edges.
Field identification & locations
Identify in the field by its waxy luster, extreme hardness (it will scratch glass), and its shell-like (conchoidal) fracture pattern. It is common in limestone or chalk beds worldwide.
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