sedimentary

Chert (often called Flint when dark)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, tan, white, or brown; Luster: Waxy to vitreous; Structure: Cryptocrystalline (grains too small to see); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.65.

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

Identified March 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of silica-rich fluids within sedimentary rock layers, often replacing carbonate minerals or filling voids. It can be found in limestone or chalk beds and spans from the Precambrian to recent geological ages.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp edges; used in modern industry for road surfacing, grinding media, and occasionally as semi-precious gemstones (jaspers) for jewelry.

Geological facts

Chert was one of the most important minerals for early humans, used for starting fires and making tools. It is so tough that it was used as 'flintlock' mechanisms in early firearms. This specimen shows a classic conchoidal fracture (shell-like curves).

Field identification & locations

Identify in the field by its hardness (it will easily scratch glass), waxy appearance, and sharp, curved edges when broken. It is commonly found in riverbeds, agricultural fields with limestone soil, and coastal chalk cliffs.

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