sedimentary

Chert Concretion with Iron Oxide Staining

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 Mohs scale; Color: Tan, brown, and grey with rusty iron staining; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 Mohs scale
Color: Tan, brown, and grey with rusty iron staining
Luster: Dull to waxy

Identified June 19, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs scale; Color: Tan, brown, and grey with rusty iron staining; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed by the precipitation of silica within sedimentary rock layers, often replacing organic material or filling voids. The exterior displays oxidation (rusting) from exposure to water and oxygen.

Uses & applications

Historically used for making stone tools (arrowheads/scrapers); currently used for landscaping, road base, and as a collector's specimen.

Geological facts

Chert is so hard it can strike sparks against steel. The nodular shape is typical of chemical precipitation in limestone or shale beds.

Field identification & locations

Identify by its waxy luster on fresh breaks, extreme hardness (cannot be scratched by a steel knife), and conchoidal (shell-like) fracture patterns. Frequently found in riverbeds or eroded limestone outcrops.