sedimentary

Chert (and Flint)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 Mohs; Color: Tan, brown, grey, cream; Luster: Waxy to vitreous; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 6
Color: Tan, brown, grey, cream
Luster: Waxy to vitreous

Identified July 15, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Tan, brown, grey, cream; Luster: Waxy to vitreous; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed through the replacement of organic material or carbonate rocks by silica-rich fluids. It often occurs as nodules in limestone or chalk beds. This specimen displays jagged conchoidal fractures typical of rapid silica precipitation.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp edges. Modern uses include road surfacing, decorative landscaping, and occasionally as a flint-starter for fires.

Geological facts

Chert is a cryptocrystalline variety of quartz. When it is dark grey or black, it is specifically referred to as flint. It has been one of the most important minerals in human history for tool-making and fire-starting.

Field identification & locations

Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and its distinctive shell-like (conchoidal) fracture patterns. Frequently found in riverbeds or eroded out of limestone cliffs.