sedimentary
Chert (variant Flint/Jasper)
Microcrystalline Opaline Silica (SiO2)
Hardness: 6.5-7 Mohs. Color: Grayish-blue to dark gray with banding. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.
- Hardness
- 6
- Color
- Grayish-blue to dark gray with banding
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs. Color: Grayish-blue to dark gray with banding. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.
Formation & geological history
Forms in marine environments by the accumulation of siliceous remains of organisms like diatoms or radiolarians, or through chemical precipitation in limestone/chalk beds. These specimens can range from Precambrian to Cenozoic in age.
Uses & applications
Historically used for stone tools and fire-starting (flint). Currently used in road construction, as an abrasive, and occasionally for decorative cabochons or landscape gravel.
Geological facts
Chert was one of the first materials traded over long distances by prehistoric humans due to its ability to be knapped into sharp edges. The blue-gray coloration often indicates the presence of organic matter or trace minerals trapped during formation.
Field identification & locations
Identify in the field by its very smooth, glass-like (conchoidal) fracture surfaces and its ability to scratch glass. It is common in limestone areas and along riverbeds. If struck with steel, it may produce sparks.
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