sedimentary
Chert (Chert / Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, tan, or brown; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Gray, tan, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, tan, or brown; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of silica-rich fluids within sedimentary rock layers, often as nodules or bedded deposits. Many deposits formed millions of years ago from the remains of silica-secreting organisms like diatoms and radiolaria.
Uses & applications
Historically used for stone tools and starting fires (flint); currently used as a road aggregate, in jewelry as cabochons, and as a landscaping material.
Geological facts
Chert has a characteristic conchoidal (shell-like) fracture that creates sharp edges, which made it the primary material for prehistoric arrowheads, scrapers, and knives.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), waxy luster, and its sharp, curved fracture edges. It is extremely common in limestone and chalk formations worldwide.
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