sedimentary
Chert (often colloquially referred to as Flint)
Chert (Cryptocrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: Gray, brown, or black; Luster: Dull to waxy; Structure: Cryptocrystalline (indistinguishable crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, brown, or black
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, brown, or black; Luster: Dull to waxy; Structure: Cryptocrystalline (indistinguishable crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed by the accumulation of silica-rich organic remains (like radiolarians or diatoms) on the ocean floor or through the chemical replacement of limestone/dolomite by silica-rich fluids. It occurs globally in geological formations ranging from the Precambrian to the present.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges. Modern uses include road surfacing material, decorative landscaping, and as a component in ceramics and glass making.
Geological facts
Chert has a conchoidal fracture, meaning it breaks like glass to create extremely sharp, durable edges. It was one of the most vital resources for Stone Age civilizations for hunting and fire-starting.
Field identification & locations
Identify in the field by its very smooth, glass-like fracture surfaces and extreme hardness (it will easily scratch steel). It is commonly found as nodules or layers within limestone and chalk beds. Collectors look for pieces with 'conchoidal' (shell-like) ripples on the break point.
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