sedimentary
Chert (Nodular or Massive Variety)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Brown, tan, grey, or greenish-grey; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Brown, tan, grey, or greenish-grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Brown, tan, grey, or greenish-grey; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by the chemical replacement of limestone/chalk by silica-rich fluids. Often formed in deep-sea environments during the Paleozoic or Mesozoic eras.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp fracture edges. Modernly used in highway construction as aggregate and occasionally as a gemstone (jasper/agate varieties) or for aquarium landscaping.
Geological facts
Chert is so hard it can create sparks when struck against steel, which made it a vital survival tool for early humans. Its extreme durability allows it to survive long-term erosion that destroys surrounding rock.
Field identification & locations
Identify in the field by its very high hardness (it will scratch glass) and its signature conchoidal (shell-like) fracture patterns. It is commonly found in limestone outcrops or as weathered river cobbles.
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