sedimentary
Chert (Nodule)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, or grayish-white; Luster: Waxy to dull; Crystal structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, or grayish-white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, or grayish-white; Luster: Waxy to dull; Crystal structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the precipitation of silica-rich fluids within sedimentary rock layers, often limestone or chalk. It can be formed biogenically from the remains of diatoms or radiolarians, or chemically from hydrothermal fluids.
Uses & applications
Historically used for making stone tools (arrowheads/scrapers) due to its sharp edges; modernly used as road aggregate, in glass manufacturing, or as decorative gravel.
Geological facts
Chert is so hard it can scratch steel. When struck against steel, it produces sparks, which led to its historical use in flintlock firearms. It is incredibly durable and resistant to weathering.
Field identification & locations
Identify in the field by its smooth, waxy texture and characteristic 'shell-like' (conchoidal) curved breaks. Found globally in sedimentary basins. Collectors look for unique coloration or 'banded' varieties.
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