Chert (Flint nodule)

Physical Properties

Hardness: 7 on Mohs scale; Color: Gray to dark gray with a weathered white/off-white cortex; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed by the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical replacement of limestone/chalk by silica in marine environments. Often found as nodules within limestone or chalk beds.

Uses & Applications

Historically used for stone tools and weapons (arrowheads) due to its sharp edges when fractured. Modernly used as a decorative stone, in road construction, and for fire-starting (flint and steel).

Geological Facts

Chert is so tough and chemically stable that it often remains as pebbles in stream beds long after the surrounding limestone carbonate has weathered away. High-quality varieties like Jasper or Agate are used as gemstones.

Field Identification & Locations

Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It is commonly found in sedimentary basins and along beaches where it has been eroded out of chalk cliffs.

Identified on: 5/7/2026

Mode: Standard