sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Brown, tan, or dark grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Hardness: 7 on Mohs scale
Color: Brown, tan, or dark grey
Luster: Waxy to dull

Identified June 26, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Brown, tan, or dark grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Formation & geological history

Formed in marine environments through the accumulation of silica-rich organic remains or chemical precipitation within limestone or chalk beds. Most common in Paleozoic and Mesozoic strata.

Uses & applications

Historically used for stone tools and weapons due to sharp edges; currently used for road surfacing, as a grinding medium, and sometimes as a minor gemstone (jasper).

Geological facts

Chert has been used by humans for over 2 million years to make arrowheads and scrapers. When struck against steel, it can produce sparks, leading to its frequent use as flint.

Field identification & locations

Identified by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture. Commonly found in riverbeds and as nodules in carbonate rocks.