sedimentary
Chert (Common Opal variant)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: White to creamy tan with waxy luster; Crystal Structure: Cryptocrystalline/Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
- Hardness
- 6
- Color
- White to creamy tan with waxy luster
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Physical properties
Hardness: 6.5-7 Mohs; Color: White to creamy tan with waxy luster; Crystal Structure: Cryptocrystalline/Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
Formation & geological history
Formed from the accumulation of silica-rich fluids in sedimentary basins or the replacement of limestone/dolomite. Often results from the remains of diatoms or radiolarians. Age can range from Precambrian to Cenozoic.
Uses & applications
Historically used for stone tools (knapping); currently used in jewelry as cabochons, as an abrasive, and by rock collectors.
Geological facts
Chert and its variants have been the primary material for arrowheads and scrapers for thousands of years due to their razor-sharp conchoidal fracture edges.
Field identification & locations
Identify in the field by its waxy appearance, inability to be scratched by steel, and 'shell-like' curved breaks. Commonly found in limestone beds and riverbeds.
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