sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Grey, brown, or tan; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Grey, brown, or tan
Luster: Waxy to vitreous

Identified May 24, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Grey, brown, or tan; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolaria) or chemical precipitation from silica-rich fluids within limestone or chalk beds.

Uses & applications

Historically used for making stone tools and starting fires; currently used in construction aggregate and as a semi-precious gemstone in jewelry (as jasper or agate varieties).

Geological facts

Chert has a conchoidal fracture pattern, meaning it breaks like glass with sharp, curved edges. Because of this, it was the primary material for spearheads and arrowheads for thousands of years.

Field identification & locations

Identified in the field by its extreme hardness (cannot be scratched by a steel knife) and its distinct shell-like fracture pattern. Commonly found as nodules or layers within sedimentary rock formations.