sedimentary
Chert (Chalcedony/Agate)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, brownish-grey, and white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brownish-grey, and white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brownish-grey, and white; 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 (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater within sedimentary host rocks. This specimen likely features a weathered cortex and internal chalcedony banding typical of flint or chert nodules.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; currently used as decorative landscaping stone, in aquarium gravel, and as a semi-precious gemstone when polished.
Geological facts
Chert is so durable that it often survives millions of years of erosion, long after the limestone or chalk it formed in has disappeared. It was one of the first materials traded by early humans over long distances.
Field identification & locations
Identify in the field by its conchoidal (shell-like) fracture, extreme hardness (it will scratch glass and steel), and waxy appearance on fresh breaks. It is commonly found in stream beds, glacial till, and embedded in limestone outcrops.
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