sedimentary
Chert (Nodule)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, brown, or grayish-white; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brown, or grayish-white
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brown, or grayish-white; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like diatoms) in limestone or chalk beds. Most specimens date from the Phanerozoic eon.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp conchoidal fracture. Industrially used as abrasive or in road construction. Small nodules are common for rock collecting and tumbling.
Geological facts
Chert is so hard it can scratch steel and was used by prehistoric humans to create fire by striking it against iron pyrite. It exists in many varieties, including flint and jasper.
Field identification & locations
Identify in the field by its waxy texture and shell-like (conchoidal) fractures. It is commonly found embedded in limestone outcrops or as weathered nodules in riverbeds across the United States and Europe.
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