sedimentary
Chert (with Iron Staining)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5–7 on Mohs scale. Color: Brown, tan, and gray. Luster: Dull to waxy. Structure: Non-crystalline mass (cryptocrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.
- Hardness
- 6
- Color
- Brown, tan, and gray
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5–7 on Mohs scale. Color: Brown, tan, and gray. Luster: Dull to waxy. Structure: Non-crystalline mass (cryptocrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the replacement of calcium carbonate in limestone by silica-rich fluids. Can date from the Precambrian to present, commonly found in Paleozoic or Mesozoic strata.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp conchoidal fracture. Modernly used in road construction, as an abrasive, and sometimes in landscaping or basic lapidary work.
Geological facts
Chert is so durable it often remains after the surrounding limestone bedrock has weathered away. It was one of the most important resources for prehistoric humans. The reddish-brown color in this specimen is caused by iron oxide impurities.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and smooth, shell-like (conchoidal) fractures. Found worldwide in riverbeds, fields, and sedimentary outcroppings. Common in the American Midwest and South.
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