sedimentary
Chert (Jasper variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Opaque brown, ochre, or tan; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
- Hardness
- 6
- Color
- Opaque brown, ochre, or tan
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Opaque brown, ochre, or tan; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolaria or diatoms) or chemical precipitation from hydrothermal fluids in sedimentary environments. Common in Paleozoic to Cenozoic strata.
Uses & applications
Historically used for stone tools/arrowheads due to its sharp conchoidal fracture. Modernly used in lapidary work, jewelry (cabochons), and as aquarium or landscaping gravel.
Geological facts
Chert is so durable that it often survives as 'river rocks' long after the surrounding limestone or shale has eroded away. Iron oxide impurities give this specific specimen its brownish/yellowish hue.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, shell-like (conchoidal) breaks. Found globally in creek beds, glacial till, and embedded in limestone cliffs.
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