Chert

Physical Properties

Hardness: 7 Mohs scale; Color: Brown, tan, gray; Luster: Waxy to dull; Crystal structure: Microcrystalline quartz (trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed by the accumulation of siliceous remains of organisms like diatoms or radiolarians, or through chemical precipitation in marine environments. Occurs in beds or as nodules within limestone or chalk layers.

Uses & Applications

Historically used for stone tools (flint knapping) and fire-starting. Currently used in jewelry (as jasper), landscaping, and as an abrasive.

Geological Facts

Chert has a characteristic conchoidal fracture, which creates sharp edges. This made it the primary material for prehistoric arrowheads, scrapers, and knives.

Field Identification & Locations

Identified in the field by its hardness (scratches glass), waxy luster, and curved, shell-like fracture surfaces. Frequently found in limestone outcrops or as smooth river pebbles.

Identified on: 5/3/2026

Mode: Standard