Chert (or Flint)

Physical Properties

Hardness: 7 on Mohs scale; Color: Gray, brown, or black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed from the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolarians) or through chemical precipitation in marine environments. It is often found as nodules in limestone or chalk beds.

Uses & Applications

Historically used for making stone tools (knapping) due to its sharp edges. Currently used in construction aggregate, as a gemstone (jasper/agate varieties), and for fire-starting in survival kits.

Geological Facts

Flint is a variety of chert that was essential to human survival for thousands of years, used for both hunting tools and starting fires through sparks. The conchoidal fracture creates edges thinner and sharper than a surgeon's steel scalpel.

Field Identification & Locations

Identified in the field by its smooth, waxy surface and its distinctive curved, shell-like (conchoidal) fractures. It is much harder than steel and will easily scratch glass. Commonly found in riverbeds or eroding out of limestone cliffs.

Identified on: 5/6/2026

Mode: Standard