sedimentary
Chert (Jasper/Agate variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5–7 (Mohs scale), Color: Variegated greens and browns, Luster: Waxy to vitreous, Crystal structure: Trigonal/Microcrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.6–2.7
- Hardness
- 6
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Physical properties
Hardness: 6.5–7 (Mohs scale), Color: Variegated greens and browns, Luster: Waxy to vitreous, Crystal structure: Trigonal/Microcrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.6–2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation within sedimentary rock layers like limestone. It often forms in marine environments and can be of various geological ages ranging from Precambrian to Cenozoic.
Uses & applications
Primary use is for lapidary work (jewelry, cabochons) and as a collector's specimen. Historically used for making stone tools (arrowheads) due to its sharp edges when fractured.
Geological facts
Chert is so durable that it often remains behind as 'lag' after the surrounding limestone or chalk has eroded away. The green coloration often comes from chlorite or iron inclusions.
Field identification & locations
Identify in the field by its smooth, waxy surface and its ability to scratch glass. It produces a distinct curved (conchoidal) fracture pattern when broken. Commonly found in riverbeds or coastal rocky outcrops.
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