Chert (and/or Flint)

Physical Properties

Hardness: 7 on Mohs scale; Color: Gray, tan, white, or brown due to impurities; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from silica-rich fluids within limestone or chalk beds. Most common in Paleozoic and Mesozoic strata.

Uses & Applications

Historically used for stone tools and weapons (arrowheads) due to its sharp conchoidal edges; used in early fire-starting kits; currently used as road aggregate or in jewelry (as jasper or agate depending on quality).

Geological Facts

Chert is so hard it can scratch glass and steel. It was one of the most vital materials for prehistoric humans globally. When struck against steel, it produces sparks used to ignite gunpowder in flintlock firearms.

Field Identification & Locations

Identify by its waxy texture and shell-like (conchoidal) fracture patterns when broken. Look for it as nodules or layers within limestone outcrops or as rounded pebbles in stream beds.

Identified on: 4/26/2026

Mode: Standard