sedimentary
Chert (Gravel pebble)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, or grey with a dull to waxy luster; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or grey with a dull to waxy luster
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or grey with a dull to waxy luster; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica in sedimentary environments, often replacing organic material or occupying voids in limestone. Can be formed from the accumulation of silica-rich shells of microscopic organisms (radiolarians/diatoms) over millions of years.
Uses & applications
Predominantly used as architectural fill, aggregate for road construction, and landscape gravel. Historically used by prehistoric humans for tool making (arrowheads/scrapers) due to its sharp edges when fractured.
Geological facts
Chert is so hard it can scratch glass and steel. It is chemically identical to quartz and amethyst but has a different internal structure. Flint is a specific dark variety of chert often found in chalk deposits.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a knife) and its smooth, shell-like (conchoidal) fracture pattern. Commonly found in riverbeds, glacial deposits, and as pebbles in gravel driveways worldwide.
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