Chert (with Quartz Vein)

Physical Properties

Hardness: 7 on Mohs scale; Color: Dark grey to brown matrix with white mineral vein; Luster: Waxy to dull matrix, vitreous vein; Crystal structure: Cryptocrystalline; 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 chemical precipitation in marine environments. The white vein formed later via hydrothermal activity or groundwater mineral deposition in rock fractures.

Uses & Applications

Historically used as stone tools (arrowheads/scrapers) due to conchoidal fracture. Currently used as road aggregate, in landscaping, and as a polishing material in rock tumblers.

Geological Facts

Chert is so hard it can actually scratch steel. When struck against steel, it can create sparks, a property shared with its variety, flint. The white band visible is a secondary quartz vein that filled a crack in the older host rock.

Field Identification & Locations

Identify in the field by its hardness (will scratch glass), smooth waxy texture on fresh breaks, and distinct conchoidal (shell-like) fracture patterns. Found globally in limestone formations.

Identified on: 4/26/2026

Mode: Standard