Chert (often called Flint)

Physical Properties

Hardness: 7 on Mohs scale; Color: Gray, tan, or brown with a waxy luster; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

Formed as nodules or layers in limestone through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater. Found in sedimentary basins worldwide.

Uses & Applications

Historically used for stone tools and making fire (striking steel). Modernly used as a road base, in construction aggregate, and occasionally as a semi-precious gemstone.

Geological Facts

Chert was one of the most important materials for early humans because its conchoidal fracture produces extremely sharp edges, ideal for spearheads and knives.

Field Identification & Locations

Identify in the field by its waxy appearance and shell-like (conchoidal) breakage patterns. It will easily scratch glass. Commonly found in limestone outcrops or as rounded pebbles in stream beds.

Identified on: 5/6/2026

Mode: Standard