sedimentary
Chert (Jasper/Flint variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale); Color: Gray, tan, and dark brown; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 6
- Color
- Gray, tan, and dark brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Gray, tan, and dark brown; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids. Commonly found in limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons due to conchoidal fracturing; modernly used as road aggregate, laboratory surfaces, or as decorative river rock.
Geological facts
Because of its hardness and ability to hold a sharp edge, it was the primary material for arrowheads and scrapers for millennia. Some varieties are called 'Flint' when found in chalk.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. Often found as nodules within sedimentary rock layers or as rounded river cobbles.
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