sedimentary
Chert (Iron-stained)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Gray-blue with orange/red iron oxide staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Gray-blue with orange/red iron oxide staining
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray-blue with orange/red iron oxide staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolarians or diatoms) or from chemical precipitation. The iron staining occurs when the rock is exposed to oxygen and water over geological time.
Uses & applications
Used historically for stone tools (knapping) due to its sharp edges; modern uses include road ballast, decorative landscaping, and occasionally lapidary work for jewelry.
Geological facts
Chert is so durable that it is often found as 'float' in stream beds long after the surrounding limestone has eroded away. Prehistoric humans specifically sought this material for spearheads and flintknapping.
Field identification & locations
Identify in the field by its characteristic smooth, shell-like (conchoidal) fracture and its ability to scratch glass. It often occurs as nodules within limestone or chalk beds.
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