sedimentary
Chert (Flint)
Microcrystalline Quartz or Cryptocrystalline Silica (SiO2)
Hardness: 7 (Mohs scale). Color: Tan, brown, and white banding. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brown, and white banding
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 (Mohs scale). Color: Tan, brown, and white banding. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
Formation & geological history
Formed as a chemical precipitate or through the replacement of limestone or chalk by silica-rich groundwater. Many specimens date from the Paleozoic and Mesozoic eras, often found in marine deposits.
Uses & applications
Historically used for stone tools and starting fires. Modern uses include road surfacing, decorative landscaping, and occasionally as minor jewelry (cabochons).
Geological facts
Chert has been used by humans for over 2 million years to create sharp-edged tools like arrowheads and knives because of its predictable conchoidal fracture patterns.
Field identification & locations
Identify in the field by its smooth, glass-like fracture surfaces and extreme hardness (it will scratch glass). Often found in riverbeds or as nodules within limestone outcrops.
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