sedimentary
Chert (Commonly known as Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brownish-tan to yellowish-gray, 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: Brownish-tan to yellowish-gray, Luster: Waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed primarily through the chemical precipitation of silica-rich fluids in marine environments or by the replacement of carbonate minerals in limestone; frequently found as nodules or layered beds in sedimentary rock sequences ranging from Precambrian to Cenozoic ages.
Uses & applications
Historically used for stone tools (arrowheads, knives) and strike-a-lights; currently used in construction aggregate, road surfacing, jewelry as cabochons, and as a raw material for decorative landscaping.
Geological facts
Chert is so hard it can scratch steel and was one of the most important minerals for early human civilizations to create sharp cutting tools; it is also highly resistant to chemical weathering.
Field identification & locations
Identified in the field by its characteristic smooth, curved (conchoidal) fracture surfaces that look like broken glass and its extreme hardness; look for it in limestone or chalk deposits globally.
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