sedimentary
Chert (varietal Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Gray to tan with a weathered tan patina; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Gray to tan with a weathered tan patina
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray to tan with a weathered tan patina; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed as nodules or layered deposits in limestone or chalk through the accumulation of siliceous remains of organisms like diatoms and sponges, or via chemical precipitation from silica-rich fluids. Can be billions of years old or relatively recent depending on the host rock.
Uses & applications
Used historically for stone tools and fire-starting (flintknapping); industrially used as an abrasive, for road surfacing, and as a raw material in ceramics/glass. Sometimes used in jewelry as cabochons.
Geological facts
Chert was one of the most important minerals for early humans because its conchoidal fracture produces razor-sharp edges perfect for arrowheads and scrapers. When struck against steel, it creates sparks used to ignite tinder.
Field identification & locations
Identify in the field by its very hard surface (scratches glass), waxy texture when freshly broken, and smooth, curved (conchoidal) fracture planes. Frequently found in limestone quarries, riverbeds, and coastal chalk cliffs.
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