Chert (Iron-rich)

Physical Properties

Hardness: 7 on Mohs scale; Color: Reddish-brown, tan, and grey; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (diatoms/radiolarians) or chemical precipitation from hydrothermal fluids. The reddish hues indicate iron oxidation during formation.

Uses & Applications

Historically used for stone tools/arrowheads. Modern uses include abrasives, decorative landscaping, and use in the manufacturing of glass and ceramics.

Geological Facts

Chert is so hard it can strike sparks against steel, making it a primary fire-starting tool for ancient civilizations. It is chemically identical to quartz but has a different microscopic structure.

Field Identification & Locations

Identify in the field by its hardness (it will scratch glass) and its sharp, shell-like (conchoidal) edges when broken. Frequently found in limestone or dolomite beds as nodules or layers.

Identified on: 4/19/2026

Mode: Standard