sedimentary
Chert (Nodule)
Chert (mainly Microcrystalline or Cryptocrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: Dark grey to black outer cortex with a waxy yellowish-brown interior; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.7.
- Hardness
- 7 on Mohs scale
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Dark grey to black outer cortex with a waxy yellowish-brown interior; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.7.
Formation & geological history
Formed in sedimentary environments, often through the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater within limestone or chalk beds. Most specimens date from the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its conchoidal fracture. Modernly used in construction aggregate, as a road base, and occasionally in jewelry (tumbled stones or cabochons).
Geological facts
Chert is so hard it can strike sparks from steel, leading to its frequent use in flintlock firearms. Some chert nodules contain perfectly preserved microfossils that are billions of years old.
Field identification & locations
Identify it in the field by its very smooth, glass-like (conchoidal) fracture surface and the presence of a weathered, chalky outer 'rind' or cortex. It is commonly found in limestone and chalk deposits worldwide, such as the White Cliffs of Dover.
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