sedimentary
Chert (Flint variety)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, brown, or waxy reddish-tan; Luster: Waxy to sub-vitreous; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Gray, brown, or waxy reddish-tan
- Luster
- Waxy to sub-vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, brown, or waxy reddish-tan; Luster: Waxy to sub-vitreous; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or the chemical replacement of limestone/chalk by silica. It occurs in sedimentary basins worldwide across varied geological ages, particularly in the Cretaceous and Paleozoic eras.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) and strike-a-lights. Modernly used as road aggregate, in glass manufacturing, and as decorative landscape stone.
Geological facts
Chert was one of the most important minerals for early human survival because its conchoidal fracture produces edges sharper than a steel scalpel. High-quality varieties like 'Flint' were foundational to the development of early weaponry and fire-starting technology.
Field identification & locations
Identify by its waxy appearance, extreme hardness (it will scratch glass and steel), and curved 'conchoidal' fracture patterns. Common in chalk cliffs, limestone outcrops, and riverbeds. Collectors look for translucent edges or unique banding.
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