sedimentary
Chert (Jasper/Agate variety)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Waxy orange, yellow, and brown; Luster: Vitreous to waxy; Crystal Structure: Cryptocrystalline (Trigonal); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- Waxy orange, yellow, and brown
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Waxy orange, yellow, and brown; Luster: Vitreous to waxy; Crystal Structure: Cryptocrystalline (Trigonal); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich microfossils or chemical precipitation in sedimentary basins. The silica often replaces other minerals or fills voids in host rock over millions of years.
Uses & applications
Predominantly used for lapidary work, jewelry, and decorative objects. Historically used for making stone tools (arrowheads and scrapers) due to its sharp conchoidal edges.
Geological facts
Chert is so durable that it survived through ancient archaeological sites for thousands of years. It can create sparks when struck against steel, which made it vital for early fire-starting and flintlock firearms.
Field identification & locations
Identify by its 'shell-like' conchoidal fracture patterns and inability to be scratched by a steel knife. Commonly found in riverbeds, glacial deposits, and as nodules within limestone or shale.
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