sedimentary
Chert (and Flint)
Microcrystalline Silica (SiO2)
Hardness: 7 (Mohs); Color: Tan, grey, brown, white; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs)
- Color
- Tan, grey, brown, white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs); Color: Tan, grey, brown, white; Luster: Waxy to dull; Structure: Cryptocrystalline; 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 chemical precipitation from groundwater within limestone or chalk beds. Most specimens represent ancient marine environments.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used in road construction, as a component in ceramics, and occasionally for flint-and-steel fire starting.
Geological facts
Chert breaks with a conchoidal fracture, producing edges sharper than a surgical scalpel. It has been used by humans for hundreds of thousands of years for survival tools.
Field identification & locations
Identify in the field by its hardness (scratches glass), smooth waxy texture on broken surfaces, and distinctive shell-like (conchoidal) curved fracture patterns. Often found as nodules within limestone outcrops.
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