sedimentary
Chert (Jasper/Agate variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5–7 (Mohs scale); Color: Dark olive green to brownish; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 6
- Color
- Dark olive green to brownish
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5–7 (Mohs scale); Color: Dark olive green to brownish; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich microfossils or precipitation from silica-rich hydrothermal fluids in sedimentary environments. Common in marine limestone and volcanic ash beds.
Uses & applications
Historically used for stone tools and arrowheads due to conchoidal fracture. Currently used in aquarium decor, costume jewelry, and lapidary tumbling.
Geological facts
Chert is so durable that it often survives intense weathering, which is why it is frequently found as rounded river pebbles or coastal stones long after the surrounding rock has eroded.
Field identification & locations
Identify by its waxy appearance and 'shell-like' (conchoidal) fracture surfaces. It can scratch glass. Common in riverbeds and gravel pits globally.
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