sedimentary
Chert (and Flint variant)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, brown, or tan in this specimen; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Gray, brown, or tan in this specimen
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, brown, or tan in this specimen; Luster: Waxy to dull; 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 by chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads, knives) due to its sharp edges; used as flint in strike-a-lights; currently used in construction aggregate and as a semi-precious gemstone for jewelry when colorful.
Geological facts
Chert is so hard it can scratch steel. When struck against steel, it produces sparks, which led to its historically vital use in flintlock firearms and starting fires.
Field identification & locations
Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. Commonly found in riverbeds or eroding out of limestone cliffs.
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