sedimentary
Chert (and Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Black, dark gray, or brown; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Black, dark gray, or brown
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Black, dark gray, or brown; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed from the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolarians) or through the precipitation of silica from groundwater within limestone or chalk beds.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) and fire-starting. Currently used as road surfacing, in architectural veneers, and for lapidary work.
Geological facts
Dark-colored chert is often called Flint. Because of its conchoidal fracture pattern, it breaks with sharp edges, making it the primary material for the Stone Age tool industry.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a steel knife), its smooth waxy surface, and the shell-like curved (conchoidal) fractures where it has chipped.
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