sedimentary
Banded Chert
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 7 on Mohs scale; Color: typically dark gray, black, or brown with lighter bands; Luster: waxy to dull; Crystal structure: hexagonal (cryptocrystalline); Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- typically dark gray, black, or brown with lighter bands
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically dark gray, black, or brown with lighter bands; Luster: waxy to dull; Crystal structure: hexagonal (cryptocrystalline); Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.65.
Formation & geological history
Formed primarily by the accumulation of siliceous remains of marine organisms (diatoms, radiolarians) or through the precipitation of silica from hydrothermal fluids in deep-sea environments. Often found in layers within limestone or as nodules in shale.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges when fractured. Modern uses include abrasives, road construction aggregate, and occasionally as semi-precious gemstones for jewelry when colorful.
Geological facts
Banded chert is often associated with the 'Iron Banded Formations' which are billions of years old and track the rise of oxygen in Earth's early atmosphere. It is so tough it can create sparks when struck against steel.
Field identification & locations
Identify in the field by its hardness (will scratch glass), smooth waxy texture, and shell-like (conchoidal) fracture patterns. It is extremely common in areas with ancient sedimentary rock layers like the Grand Canyon or the Appalachian Mountains.
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