sedimentary
Chert (Nodule)
Cryptocrystalline Silica (SiO2)
Hardness: 6.5–7 on Mohs scale. Color: Dark brown to black with a tan/brown weathered cortex. Luster: Waxy to dull. Crystal Structure: Cryptocrystalline (microscopic quartz). Cleavage: None (shows conchoidal fracture). Specific gravity: 2.65.
- Hardness
- 6
- Color
- Dark brown to black with a tan/brown weathered cortex
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5–7 on Mohs scale. Color: Dark brown to black with a tan/brown weathered cortex. Luster: Waxy to dull. Crystal Structure: Cryptocrystalline (microscopic quartz). Cleavage: None (shows conchoidal fracture). Specific gravity: 2.65.
Formation & geological history
Formed through the precipitation of silica in sedimentary environments, often replacing calcium carbonate in limestone. Most specimens range from the Proterozoic to Cenozoic eras (hundreds of millions of years old). It often forms as nodules or beds in marine environments from the remains of silica-shelled organisms like diatoms or radiolarians.
Uses & applications
Used historically for stone tools (arrowheads, scrapers) due to its sharp conchoidal fracture. Modern uses include road construction aggregate, abrasives, and as a low-cost gemstone for tumbling and jewelry (frequently sold as 'Jasper' or 'Flint').
Geological facts
Chert is one of the most durable sedimentary rocks. It was the primary technological resource for prehistoric humans for millions of years. When struck against steel, it can create a spark, leading to its common name 'flint' in specific contexts.
Field identification & locations
Identify in the field by its smooth, glass-like internal fracture surfaces and its ability to scratch glass. It often has a rough, light-colored outer 'rind' or cortex. Found globally in limestone and chalk deposits, and frequently in riverbeds as water-worn cobbles.
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