Chert with Quartz/Calcite Vein

Physical Properties

Hardness: 7 (Mohs scale); Color: Dark grey to black matrix with a yellowish-white vein; Luster: Waxy to dull (glassy when wet); Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. The vein formed later through hydrothermal activity where mineral-rich water filled a fracture.

Uses & Applications

Used historically for stone tools and arrowheads due to its sharp edges; currently used for road surfacing, decorative landscaping, and as a collector's specimen.

Geological Facts

Black chert is often referred to as 'flint'. The 'vein' seen here is a signature of tectonic stress where the brittle rock cracked and was later healed by secondary mineral fluids.

Field Identification & Locations

Identify by its extreme hardness (scratches glass), smooth waxy feel, and characteristic shell-like (conchoidal) fracture pattern. Commonly found in riverbeds and coastal areas.

Identified on: 4/28/2026

Mode: Standard