sedimentary

Chert (Flint)

Chert (Microcrystalline or cryptocrystalline Quartz, SiO2)

Hardness: 6.5–7 on Mohs scale. Color: Highly variable, often tan, grey, or white with waxy to dull luster. Crystal structure: Cryptocrystalline. Fracture: Conchoidal (shell-like) with sharp edges. Opaque to translucent.

Hardness: 6

Identified March 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. It often occurs as nodules in limestone or chalk beds and can date from the Precambrian to the present.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp fracture. Currently used in road construction (aggregate), as a historical fire-starter (flint and steel), and sometimes in jewelry as cabochons.

Geological facts

Chert was one of the most important minerals to early humans for tool-making. It is so tough that it was frequently used as a ‘strike-a-light’ to create sparks for fire long before the invention of matches.

Field identification & locations

Identify in the field by its extreme hardness (will scratch steel) and its distinctive smooth, curved conchoidal fracture. Commonly found in sedimentary basins, riverbeds, and as nodules in limestone outcrops.

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