sedimentary
Chert (Vuggy or Chalcedony)
Cryptocrystalline Quartz (SiO2)
Hardness: 7.0 Mohs. Color: Tan, brown, and ochre. Luster: Waxy to dull. Structure: Microcrystalline with irregular, curvy habit. Cleavage: None (conchoidal fracture).
- Hardness
- 7
- Color
- Tan, brown, and ochre
- Luster
- Waxy to dull
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Physical properties
Hardness: 7.0 Mohs. Color: Tan, brown, and ochre. Luster: Waxy to dull. Structure: Microcrystalline with irregular, curvy habit. Cleavage: None (conchoidal fracture).
Formation & geological history
Chemical precipitation of silica from groundwater or within sedimentary rock cavities. Often forms as nodules in limestone or as thin beds. Ages range from Precambrian to Cenozoic.
Uses & applications
Historically used for stone tools due to sharp edges; currently used for decorative stone landscaping, aquarium decor, and as a collector's specimen.
Geological facts
This specific piece shows a 'vuggy' or 'nodular' habit where silica filled a weirdly shaped void in a host rock. Chert is so hard it can create sparks when struck against steel.
Field identification & locations
Identified by its hardness (cannot be scratched by a steel nail) and waxy texture. Commonly found in riverbeds or eroded out of limestone cliffs.
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