sedimentary
Chert (vuggy variety)
Microcrystalline silica (SiO2)
Hardness (Mohs scale): 7; Color: Tan, beige, or yellowish-white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Color
- Tan, beige, or yellowish-white
- Luster
- Waxy to dull
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Physical properties
Hardness (Mohs scale): 7; Color: Tan, beige, or yellowish-white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. The 'vuggy' holes are often created by the weathering out of fossils or softer mineral inclusions over geological time.
Uses & applications
Historically used for stone tools and flintknapping. In modern times, used in road construction, as architectural aggregate, and as a low-cost specimen for educational collecting.
Geological facts
Chert is nearly chemically identical to quartz but has a much finer grain size. It is extremely durable and has been used by humans since the Stone Age for spearheads and scrapers because of its sharp edges.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its waxy feel, and its distinct conchoidal (shell-like) fracture patterns. It is common in limestone and dolomite formations worldwide.
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