sedimentary
Chert (specifically Gray Chert/Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale. Color: Gray, tan, and creamy white. Luster: Waxy to dull. Crystal structure: Trigonal (microscopic). Cleavage: None, exhibits conchoidal fracture. Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, and creamy white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale. Color: Gray, tan, and creamy white. Luster: Waxy to dull. Crystal structure: Trigonal (microscopic). Cleavage: None, exhibits conchoidal fracture. Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. Often found as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) and strike-a-lights. Industrially used as an abrasive and in glass making. Common in landscaping gravel.
Geological facts
Chert was one of the most important minerals for early humans because its conchoidal fracture creates extremely sharp edges. It is chemically identical to quartz and amethyst but has a different internal structure.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) breakage patterns. Found globally, especially in sedimentary basins.
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