sedimentary
Chert (Flint)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, tan, brown, or black; Luster: Dull to waxy; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Gray, tan, brown, or black
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, tan, brown, or black; Luster: Dull to waxy; Crystal Structure: Trigonal (microscopic); 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 through the chemical replacement of limestone/dolomite by silica-rich fluids. Found in marine sedimentary environments.
Uses & applications
Historically used for stone tools (arrowheads, knives) and strike-a-lights; currently used in construction aggregate and as a semi-precious gemstone in jewelry when colorful.
Geological facts
Because it breaks with a conchoidal fracture, it creates edges sharper than a surgeon's steel scalpel. It was one of the most vital materials for prehistoric human survival.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by steel), waxy texture on fresh surfaces, and smooth, curved shell-like fracture patterns. Common in limestone outcrops.
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