Chert (Flint)

Physical Properties

Hardness: 7 Mohs scale; Color: Dark grey to black with white weathered cortex; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

Formed via chemical precipitation of silica-rich fluids or the accumulation of siliceous skeletal remains (radiolarians/diatoms) in marine environments. This specific piece shows a weathered outer layer (cortex).

Uses & Applications

Used historically for stone tools and fire-starting; currently used as road aggregate, in jewelry as semi-precious stones, and in the ceramic industry.

Geological Facts

Flint is a variety of chert that occurs primarily in chalk or marly limestone deposits. It was the most important material for early human toolmaking during the Stone Age due to its sharp conchoidal edges.

Field Identification & Locations

Identified by its extremely smooth, shell-like fracture surfaces and ability to scratch glass. Commonly found in limestone outcrops, riverbeds, and coastal areas like the White Cliffs of Dover.

Identified on: 5/20/2026

Mode: Standard