Chert (Quartz Variety)

Physical Properties

Hardness: 7 (Mohs scale); Color: Tan, beige, white, or gray; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or the chemical replacement of limestone/dolomite by silica-rich fluids. Can be found in rock layers dating from the Precambrian to the present.

Uses & Applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp conchoidal edges; currently used as construction aggregate, road base, and occasionally in jewelry as 'jasper'.

Geological Facts

Chert is so hard it can create sparks when struck against steel, which made it essential for flintlock firearms and early fire-starting. It is often referred to as 'Flint' when found in chalk or marly limestone.

Field Identification & Locations

Identify by its waxy appearance, extreme hardness (it will scratch glass), and smooth, curved (conchoidal) fracture surfaces. Commonly found in stream beds, gravel pits, and as nodules within limestone outcrops.

Identified on: 4/27/2026

Mode: Standard