sedimentary
Chert Nodule (Commonly known as a Flint or River Cobble)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, or light brown; Luster: Dull to waxy; Structure: Microcrystalline with conchoidal (shell-like) fracture patterns; Cleavage: None.
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or light brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or light brown; Luster: Dull to waxy; Structure: Microcrystalline with conchoidal (shell-like) fracture patterns; Cleavage: None.
Formation & geological history
Formed through the replacement of limestone or carbonate sediment by silica-saturated groundwater. These nodules often accumulate in sedimentary basins or are weathered out into riverbeds and glacial deposits; geological age can range from Precambrian to Recent.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to sharp edges; currently used in construction aggregate, road base, and as decorative river rock in landscaping.
Geological facts
Chert is so chemically stable that it often survives millions of years of erosion that destroys surrounding limestone. When struck with steel, it can create sparks, which led to its historical use in flintlock firearms.
Field identification & locations
Identify by its 'skin' or cortex (the chalky outer layer) and smooth, rounded shape from water tumbling. Can be found in riverbeds, beaches, or gravel pits. Test by attempting to scratch glass; chert will leave a scratch.
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