sedimentary
Chert with Quartz Vein
Microcrystalline Quartz (SiO2) with Macrocrystalline Quartz
Hardness: 7 on Mohs scale. Color: Dark grey to black (chert) and white/tan (quartz). Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None, exhibits conchoidal fracture.
- Hardness
- 7 on Mohs scale
- Color
- Dark grey to black (chert) and white/tan (quartz)
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale. Color: Dark grey to black (chert) and white/tan (quartz). Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None, exhibits conchoidal fracture.
Formation & geological history
Formed by the precipitation of silica from seawater or volcanic ash. The white quartz portion represents a hydrothermal vein where silica-rich fluids filled a fracture in the darker host rock at a later geological time.
Uses & applications
Used historically for stone tools due to sharp edges when fractured. Modernly used in landscaping, as a gemstone (jaspers), and as an abrasive in industrial applications.
Geological facts
Black chert is often called flint. This specimen is a classic example of a 'veined' rock, where tectonic stress created a crack that was later healed by mineral-rich water.
Field identification & locations
Identified by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) breaking pattern. Common in riverbeds and limestone formations worldwide.
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