Chert (with Quartz veins)

Physical Properties

Hardness: 7 on Mohs scale; Color: Dark grey to black with white/yellowish veins; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & Geological History

Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or inorganic precipitation in marine environments; age varies from Precambrian to Cenozoic. The veins formed later via hydrothermal activity.

Uses & Applications

Used historically for stone tools (arrowheads); currently used in road construction, as jewelry (when polished), and as an abrasive.

Geological Facts

Chert is extremely durable and chemically stable. Dark varieties like this are often called flint. It is one of the most common materials used by prehistoric humans for tool-making due to its sharp edges when fractured.

Field Identification & Locations

Identify in the field by its hardness (scratches glass), smooth waxy feel, and characteristic curved 'conchoidal' fractures. Commonly found in riverbeds and coastal areas as rounded cobbles.

Identified on: 5/1/2026

Mode: Standard