sedimentary
Chert with Iron Oxide Staining
Microcrystalline Quartz (SiO2) with Limonite/Goethite staining
Hardness: 7 (Mohs scale), Color: White/Grey interior with rusty orange/brown surface, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture)
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Physical properties
Hardness: 7 (Mohs scale), Color: White/Grey interior with rusty orange/brown surface, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture)
Formation & geological history
Formed from the accumulation of silica-rich organic remains or chemical precipitation in marine or groundwater environments. The orange staining is a secondary process involving the weathering of iron-rich minerals.
Uses & applications
Historically used for making stone tools (flint knapping) due to its sharp edges; used today as a decorative rock or road aggregate.
Geological facts
Chert was one of the most important minerals for early humans, used for spearheads and fire-starting. The orange crust is essentially a form of biological or chemical rust.
Field identification & locations
Identified in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture pattern. Commonly found in limestone beds.
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