sedimentary
Chert (and/or Chalcedony)
Silicon dioxide (SiO2)
Hardness: 6.5-7 (Mohs scale); Color: White, tan, cream, or light grey; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 6
- Color
- White, tan, cream, or light grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: White, tan, cream, or light grey; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Forms through the precipitation of silica from groundwater or the accumulation of siliceous biological remains (such as diatoms or radiolarians) in sedimentary basins. This specimen likely originates from limestone or chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp conchoidal fracture. Modernly used as an abrasive, in construction aggregate, or as a decorative specimen for collectors.
Geological facts
Chert is so hard it can scratch steel. It was one of the most important minerals for early human survival because it can be chipped into tools with edges sharper than a steel razor.
Field identification & locations
Look for a glassy/waxy luster and shell-like (conchoidal) fractures on broken edges. It will not be scratched by a steel knife. Commonly found as nodules in limestone regions worldwide.
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