sedimentary
Chert
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brown, tan, cream, and white mottling, Luster: Dull to waxy, Crystal Structure: Hexagonal (microscopic), Cleavage: None (conchoidal fracture)
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown, tan, cream, and white mottling, Luster: Dull to waxy, Crystal Structure: Hexagonal (microscopic), Cleavage: None (conchoidal fracture)
Formation & geological history
Formed by the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolaria) or by chemical precipitation from silica-rich groundwater replacing other minerals in carbonate rocks.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp fracture edges. Modernly used as road base, decorative landscaping stone, and occasionally in jewelry as jasper.
Geological facts
Chert is so hard it can strike sparks against steel, a property that was used for centuries in flintlock firearms and for starting fires. Variations are often called Flint (if dark/black) or Jasper (if red/yellow).
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its curved, shell-like (conchoidal) fracture surfaces. Commonly found in limestone and dolomite beds as nodules or layers.
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