sedimentary
Chert (Iron-stained)
Microcrystalline Quartz (SiO2)
Hardness: 7 Mohs scale; Color: Brown, tan, reddish-orange due to iron oxides; Luster: Dull to waxy; Structure: Cryptocrystalline/Microcrystalline; Cleavage: None (conchoidal fracture).
- Hardness
- 7 Mohs scale
- Color
- Brown, tan, reddish-orange due to iron oxides
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 Mohs scale; Color: Brown, tan, reddish-orange due to iron oxides; Luster: Dull to waxy; Structure: Cryptocrystalline/Microcrystalline; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed primarily as a chemical precipitate from silica-rich fluids in sedimentary environments. Common in marine deposits where silica from radiolarians or sponges accumulates.
Uses & applications
Historically used for stone tools due to conchoidal fracturing. Modernly used as construction aggregate, decorative landscaping stone, and occasionally for lapidary work.
Geological facts
Chert is so durable that it often survives as rounded pebbles in streambeds long after the surrounding rock has weathered away. It is often found as nodules inside limestone.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds and glacial till.
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