sedimentary
Chert (Chalcedony/Agate)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale. Color: Tan, brown, and grey. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Tan, brown, and grey
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: Tan, brown, and grey. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
Formation & geological history
Formed from the accumulation of silica-rich fluids in sedimentary layers or through the chemical replacement of limestone/dolomite. This specimen shows typical nodules common in many geological eras from Paleozoic to Cenozoic.
Uses & applications
Historically used for stone tools (arrowheads). Modern uses include landscaping, tumbling for collectors, and as a raw material for silicon production.
Geological facts
Chert is so hard it can create sparks when struck against steel. It was one of the most vital materials for early humans to create survival tools.
Field identification & locations
Look for a waxy appearance and shell-like (conchoidal) curves where the rock has fractured. It will easily scratch glass. Commonly found in riverbeds and limestone outcrops.
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