sedimentary
Chert (Agatized/Chalcedony variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs); Color: Tan, brown, white, and waxy grey; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 6
- Color
- Tan, brown, white, and waxy grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Tan, brown, white, and waxy grey; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in sedimentary basins. The silica often replaces original limestone or organic material over millions of years.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp conchoidal fracture. Today used for lapidary work, decorative landscaping, and as a source of silica in glass manufacturing.
Geological facts
Chert is so durable that it can survive long periods of erosion and travel great distances in riverbeds. Ancient civilizations prized certain colors of chert for trade across continents.
Field identification & locations
Identify by its waxy luster, extreme hardness (it will scratch glass), and distinctive curved, shell-like (conchoidal) fractures. Often found in limestone outcrops or as rounded river cobbles.
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