sedimentary
Chert (often called Flint for darker varieties)
Silicon Dioxide (SiO2) - Cryptocrystalline Quartz
Hardness: 7 (Mohs scale); Color: Gray, brown, or tan (as seen in image); Luster: Waxy to dull; Crystal structure: Trigonal (microscopic/cryptocrystalline); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Gray, brown, or tan (as seen in image)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, brown, or tan (as seen in image); Luster: Waxy to dull; Crystal structure: Trigonal (microscopic/cryptocrystalline); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation in marine or groundwater environments. It typically occurs as nodules or layered deposits in limestone or chalk beds.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to sharp edges; currently used as road aggregate, in glass manufacturing, and occasionally in jewelry as cabochons or tumbled stones.
Geological facts
Because of its harness and conchoidal fracture, it was the primary material for the Stone Age tool industry. It can produce sparks when struck against steel, which made it vital for fire-starting and early firearms (flintlocks).
Field identification & locations
Identify in the field by its smooth, waxy surface and curved, shell-like (conchoidal) fractures where it has chipped. It will easily scratch glass. Commonly found in riverbeds, gravel pits, and eroding out of limestone outcrops.
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