sedimentary
Chert (Iron-stained)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Mottled gray and tan with heavy brown/orange iron oxide staining; Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
- Hardness
- 7 on Mohs scale
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Mottled gray and tan with heavy brown/orange iron oxide staining; Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater. Age can range from Precambrian to Cenozoic depending on the local geology.
Uses & applications
Historically used for stone tools (arrowheads) due to its sharp edges; modern uses include road aggregate, decorative landscaping, and occasionally lapidary work if highly figured.
Geological facts
Chert is so durable that it often remains as 'float' in stream beds long after the surrounding limestone or chalk has weathered away. Iron-rich varieties are sometimes referred to as 'Jasper' if they are fully opaque.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its curved, shell-like fracture pattern. Commonly found in riverbeds, glacial till, and as nodules within carbonate rock layers.
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