sedimentary
Chert (Jasper/Agate variety)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: tan, brown, and white banding, Luster: waxy to dull, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6-2.65
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: tan, brown, and white banding, Luster: waxy to dull, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6-2.65
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolaria or diatoms) or secondary precipitation from hydrothermal fluids in sedimentary environments. Can be found in rocks ranging from the Precambrian to Cenozoic eras.
Uses & applications
Commonly used for lapidary work (jewelry, cabochons), primitive tool making (flint knapping), and sometimes as industrial abrasives or decorative landscaping stones.
Geological facts
Chert was one of the most important minerals for early humans because its conchoidal fracture allows it to break into razor-sharp edges for spears and knives. When light brown or reddish, it is often called Jasper.
Field identification & locations
Identify in the field by its hardness (it should scratch glass and steel) and its shell-like (conchoidal) breakage patterns. Look for it in limestone or chalk beds as nodules or along stream beds as tumbled river rocks.
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