sedimentary
Chert (Flint)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Black or dark grey core with tan/brown weathering rind (patina), Luster: Waxy to dull, Structure: Cryptocrystalline/microcrystalline, Cleavage: None (conchoidal fracture)
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Physical properties
Hardness: 7 (Mohs scale), Color: Black or dark grey core with tan/brown weathering rind (patina), Luster: Waxy to dull, Structure: Cryptocrystalline/microcrystalline, Cleavage: None (conchoidal fracture)
Formation & geological history
Formed as nodules or layers in limestone or chalk through the accumulation of silica-rich organisms or chemical precipitation from groundwater. This specimen shows a thick weathering rind indicating long-term exposure to the elements.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Modern uses include abrasives and light construction aggregate.
Geological facts
Black chert is often specifically called flint. Its conchoidal fracture creates edges sharper than a steel scalpel. The tan outer layer is called a 'cortex' and is the result of chemical weathering of the silica.
Field identification & locations
Identify in the field by its glass-like 'shell-shaped' fractures and ability to scratch glass. It is commonly found in riverbeds or within sedimentary rock outcrops.
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