sedimentary

Chert

Chert (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, tan, or brown with a white weathered cortex; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Gray, tan, or brown with a white weathered cortex
Luster: Waxy to dull

Identified March 3, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Gray, tan, or brown with a white weathered cortex; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed in marine environments through the accumulation of silica-rich microfossils or chemical precipitation from hydrothermal fluids, often found as nodules in limestone or chalk beds.

Uses & applications

Historically used for stone tools and weapons (flintknapping); currently used as a road aggregate, in jewelry (as jasper or agate), and for starting fires (flint).

Geological facts

Chert is essentially a form of microcrystalline quartz; when it occurs in dark gray or black colors, it is specifically referred to as flint. It was one of the most vital materials for prehistoric survival.

Field identification & locations

Identified in the field by its sharp, glass-like conchoidal fractures and ability to scratch glass. Often found in chalk cliffs, limestone outcrops, or as water-worn pebbles in riverbeds.