sedimentary
Chert (variations include Flint/Novaculite)
Chert (formula: SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, buff, or white, Luster: dull to waxy/greasy, Crystal structure: microcrystalline/cryptocrystalline quartz, Cleavage: none (conchoidal fracture), Specific gravity: 2.65.
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, buff, or white, Luster: dull to waxy/greasy, Crystal structure: microcrystalline/cryptocrystalline quartz, 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 the replacement of limestone/chalk by silica-rich fluids. Can be found in geological formations ranging from the Precambrian to present day.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used as road aggregate, in abrasive manufacturing, and occasionally as a low-cost gemstone for lapidary work.
Geological facts
Chert is so hard it can produce sparks when struck against steel, which made it the primary material for flintlock firearms. Its conchoidal fracture allows it to be knapped into edges sharper than a surgical scalpel.
Field identification & locations
Identify in the field by its extreme hardness (scratches glass), smooth conchoidal (shell-like) fracture patterns, and dull earthy appearance compared to glass. Found globally in nodules within limestone or chalk beds.
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