sedimentary
Chert (Chalcedony/Agate variety)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, or grayish; Luster: Waxy to vitreous; Crystal structure: Hexagonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, or grayish
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, or grayish; Luster: Waxy to vitreous; Crystal structure: Hexagonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich fluids in sedimentary basins or by the replacement of limestone. Can occur as nodules or bedded deposits. Geologically, these can range from Precambrian to Holocene.
Uses & applications
Used historically for stone tools due to its sharp edges; used today as decorative stones, gemstones (agates), and in high-quality abrasive materials.
Geological facts
Ancient humans used chert for thousands of years to create arrowheads, scrapers, and knives because it breaks with a conchoidal fracture, creating razor-sharp edges.
Field identification & locations
Identify in the field by its waxy appearance and shell-like (conchoidal) breaking pattern. It is commonly found in limestone deserts or riverbeds. It is hard enough to scratch glass.
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