sedimentary
Chert (variably Oolitic or Fossiliferous)
Sedimentary Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, white, or cream; Luster: Waxy to dull; Structure: Cryptocrystalline with visible rounded ooids or fossil impressions; Cleavage: None (conchoidal fracture).
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, white, or cream
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, white, or cream; Luster: Waxy to dull; Structure: Cryptocrystalline with visible rounded ooids or fossil impressions; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the chemical precipitation of silica-rich fluids within existing limestone or chalk beds, often replacing carbonate material. These sediments usually date from the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for making stone tools (arrowheads). Modern uses include landscaping, road aggregate, and as a semi-precious gemstone (Jasper/Agate variants) when polished.
Geological facts
This specimen appears to contain ooids or fossilized remains, common in cherts from the Midwestern United States. Chert is so durable it often survives long-distance transport via rivers and glaciers.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel nail) and waxy appearance. Commonly found in riverbeds, limestone quarries, and gravel pits throughout the Great Lakes and Mississippi Valley regions.
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