sedimentary

Chert (Gravel pebble)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

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

Identified March 18, 2026More sedimentary →

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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.