sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, brown, and grey; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Tan, brown, and grey
Luster: Dull to waxy

Identified June 10, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Tan, brown, and grey; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed by the precipitation of silica from aqueous fluids or the accumulation of siliceous remains of marine organisms. Commonly found in sedimentary basins and as nodules in limestone or chalk.

Uses & applications

Historically used for stone tools (knapping) and fire-starting materials. Occasionally used as road metal or in landscaping.

Geological facts

Chert has been used by humans for millions of years to make arrowheads and scrapers because of its sharp conchoidal edges. Flint is the most well-known variety of chert.

Field identification & locations

Identify in the field by its extreme hardness (cannot be scratched by steel) and smooth, shell-like fracture surfaces. Common in river beds and sedimentary outcrops.