sedimentary
Chert (Flint Nodules)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs), Color: Dark grey to brown/tan, Luster: Waxy to dull, Crystal Structure: Microcrystalline (Trigonal), Cleavage: None (Conchoidal fracture), SG: 2.65
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Physical properties
Hardness: 7 (Mohs), Color: Dark grey to brown/tan, Luster: Waxy to dull, Crystal Structure: Microcrystalline (Trigonal), Cleavage: None (Conchoidal fracture), SG: 2.65
Formation & geological history
Formed by the accumulation of siliceous remains of organisms (radiolaria or diatoms) or chemical precipitation from groundwater within sedimentary rock layers, often limestone or chalk.
Uses & applications
Historically used for stone tools and starting fires (flint); currently used in road construction, as jewelry (when patterned), and as high-quality building stone.
Geological facts
Chert was one of the most important minerals for early humans because its conchoidal fracture produces extremely sharp edges, ideal for arrowheads and knives.
Field identification & locations
Identify by its waxy appearance, extreme hardness (scratches glass), and the way it breaks into smooth curved surfaces. Commonly found in stream beds or weathering out of limestone.
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