sedimentary
Chert (often called Flint when dark)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, white, or gray (variable due to impurities); Luster: Waxy to dull; Crystal structure: Trigonal (microscopic fibers); Cleavage: None (conchoidal fracture); Specific gravity: 2.6
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, white, or gray (variable due to impurities)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, white, or gray (variable due to impurities); Luster: Waxy to dull; Crystal structure: Trigonal (microscopic fibers); Cleavage: None (conchoidal fracture); Specific gravity: 2.6
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation from silica-rich fluids within limestone or chalk beds. Often occurring as nodules or beds in sedimentary strata dating back billions of years to the present.
Uses & applications
Historically used for stone tools (arrowheads/knives) due to sharp edges; currently used for road surfacing, as a component in glass making, and as laboratory grinding media (flint pebbles).
Geological facts
Because of its conchoidal fracture, it breaks into razor-sharp edges. It was one of the most vital resources for prehistoric humans. It can also produce sparks when struck against steel, leading to its name 'flint' in specific contexts.
Field identification & locations
Identify by its 'shell-like' (conchoidal) fracture pattern and its ability to scratch glass. It is commonly found in chalk or limestone outcrops, or as smooth pebbles in riverbeds and on beaches. Collectors look for unique colors or fossil inclusions.
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