sedimentary
Chert (Black Chert/Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Black to dark gray; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Black to dark gray
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Black to dark gray; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); 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 in marine environments. Often found as nodules or layers within limestone or chalk beds. Geological age can range from Precambrian to Holocene.
Uses & applications
Historically used for stone tools and weapons due to sharp edges; currently used as a road aggregate, in glass manufacturing, and occasionally in jewelry as low-cost 'black onyx' substitutes.
Geological facts
Black chert is often specifically called 'flint' when found in chalk or marly limestone. It produces sparks when struck against steel, a property that led to its famous use in flintlock firearms.
Field identification & locations
Identify in the field by its characteristic conchoidal (shell-like) fracture and high hardness (it will scratch glass). Commonly found in riverbeds, agricultural fields over limestone bedrock, and coastal cliffs.
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