sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 Mohs; Color: Grayish core with tan/yellow weathered cortex; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 Mohs
Color: Grayish core with tan/yellow weathered cortex
Luster: Waxy to vitreous

Identified July 3, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Grayish core with tan/yellow weathered cortex; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed by the precipitation of silica in sedimentary environments, often replacing carbonate material or accumulating as radiolarian/diatomaceous ooze on the sea floor during various geological eras from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) and fire-starting (flint and steel). Currently used as aggregate, decorative stone, and in jewelry (as jasper or agate varieties).

Geological facts

Chert has a conchoidal fracture which creates extremely sharp edges; ancient obsidian and chert tools can be sharper than modern surgical steel scalpels. The weathered outer layer is known as the 'cortex'.

Field identification & locations

Identify in the field by its waxy luster, hardness (it will scratch glass), and the way it breaks into smooth, curved surfaces (conchoidal fracture). Often found as nodules in limestone or as water-worn pebbles in stream beds.