Chert / Flint

Physical Properties

Hardness: 7 on Mohs scale; Color: Grey, black, or brown; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6

Formation & Geological History

Formed in sedimentary rocks where microcrystalline silica replaces organic material or precipitates from fluids, often in marine environments within limestone or chalk beds.

Uses & Applications

Historically used for stone tools (arrowheads) and fire-starting; currently used as road aggregate, in glass manufacturing, and as a semi-precious gemstone when colorful (Jasper).

Geological Facts

Flint is a variety of chert that was the first 'utility' mineral for humans, used to create fire and sharp blades during the Stone Age due to its ability to fracture into razor-sharp edges.

Field Identification & Locations

Identify by its waxy appearance and conchoidal (curved, glass-like) fracture patterns. It is extremely hard and will scratch glass easily.

Identified on: 5/18/2026

Mode: Standard