Chert (Iron-stained)

Physical Properties

Hardness: 7 on Mohs scale; Color: Black, brown, and ochre/yellow; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from groundwater. This specimen shows a weathered 'patina' or rind with iron-oxide staining.

Uses & Applications

Historically used for making stone tools (arrowheads/scrapers) due to conchoidal fracture. Today used in decorative landscaping, construction aggregate, and as a collector's specimen.

Geological Facts

Chert is so hard it can scratch steel. When struck against steel, it can produce sparks, which is why darker varieties are often referred to as flint and were used in flintlock firearms.

Field Identification & Locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its extreme hardness. Often found as nodules within limestone or chalk beds.

Identified on: 4/20/2026

Mode: Standard