sedimentary
Chert (Black Flint)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs). Color: Black to dark grey. Luster: Waxy to dull. Crystal Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.
- Hardness
- 6
- Color
- Black to dark grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs). Color: Black to dark grey. Luster: Waxy to dull. Crystal Structure: Cryptocrystalline. Cleavage: None (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 from groundwater within limestone or chalk beds. Often Cretaceous or Paleozoic in age.
Uses & applications
Historically used for stone tools and weapons. Modern uses include road surfacing, decorative landscaping, and as a component in ceramics.
Geological facts
Black chert is often called flint. Because of its conchoidal fracture, it breaks into razor-sharp edges, making it a primary material for prehistoric survival technologies like arrowheads.
Field identification & locations
Identify by its 'shell-like' curved fractures, high hardness (cannot be scratched by steel), and frequent occurrence as nodules within softer sedimentary host rocks like limestone.
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