sedimentary
Chert Nodule
Chert (Microcrystalline Silicon Dioxide, SiO2)
Hardness: 7 (Mohs scale); Color: tan, brown core with a white weathered cortex; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.6
- Hardness
- 7 (Mohs scale)
- Color
- tan, brown core with a white weathered cortex
- Luster
- waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: tan, brown core with a white weathered cortex; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.6
Formation & geological history
Formed by the precipitation of silica within limestone or chalk beds, often through the replacement of organic materials or minerals by groundwater. This specimen shows a thick weathered rind or cortex around a dense silica core.
Uses & applications
Historically used for making stone tools (flint knapping) due to its sharp conchoidal fracture. Today used as a common construction aggregate and for geological study.
Geological facts
Chert was one of the most important minerals for early humans, used to create arrowheads, scrapers, and fire-strikers. This specific specimen is a river-worn nodule where the outer white layers represent the boundary of the original mineral growth in the matrix rock.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife), waxy internal appearance, and smooth, curved fracture surfaces. Found worldwide in riverbeds and sedimentary outcrops.
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