sedimentary
Chert (Flint)
Chert (Microcrystalline Quartz, SiO2)
Hardness: 6.5-7 on Mohs scale; Color: typically tan, brown, or gray; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.65.
- Hardness
- 6
- Color
- typically tan, brown, or gray
- Luster
- waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: typically tan, brown, or gray; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.65.
Formation & geological history
Formed by the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or by chemical precipitation from silica-rich groundwater within limestone or chalk beds. Distribution is worldwide across various geological ages.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) and starting fires; currently used in construction aggregate and occasionally as ornamental stones.
Geological facts
Chert is so hard it can scratch steel. When struck against steel, it produces sparks due to its hardness and micro-structure, which led to its use in flintlock firearms and early fire-starting kits.
Field identification & locations
Identify in the field by its smooth, waxy surface and sharp, curved (conchoidal) edges when broken. It is often found as nodules or layers within limestone. Common in riverbeds and coastal areas.
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