sedimentary
Chert / Chalcedony
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: White, cream, tan; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 (Mohs scale)
- Color
- White, cream, tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: White, cream, tan; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in sedimentary basins. Often associated with limestone or found as nodules in chalk deposits.
Uses & applications
Historically used for stone tools (flintknapping). Modern uses include construction aggregate, decorative stones, and some high-quality varieties are used in lapidary work.
Geological facts
Chert was one of the most important minerals for early humans because of how it breaks with sharp edges, allowing it to be shaped into arrowheads, scrapers, and knives.
Field identification & locations
Identify by its hardness (scratches glass), waxy luster, and conchoidal (shell-like) fracture pattern. Commonly found in riverbeds, gravel pits, and coastal cliffs.
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