sedimentary
Chert (Lightly weathered variety)
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 7 (Mohs scale); Color: Buff, tan, and grayish-brown with white spotting; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Buff, tan, and grayish-brown with white spotting
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Buff, tan, and grayish-brown with white spotting; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or the chemical precipitation of silica in marine environments. It often occurs as nodules or layers within limestone or chalk formations.
Uses & applications
Historically used for stone tools (lithics) due to its sharp conchoidal fracture. In modern times, it is used as road metal, in concrete aggregate, and occasionally as a low-cost lapidary material for jewelry like cabochons.
Geological facts
Chert is so hard it can create sparks when struck against steel, a property used by ancient humans to start fires. It is nearly identical to flint, although 'flint' is often reserved for specimens found in chalk.
Field identification & locations
Identify in the field by its hardness (it scratches glass easily) and its tendency to break into sharp, curved 'shell-like' edges. Common in riverbeds and limestone outcrops globally. Tips for collectors: look for 'slick' textures compared to surrounding grainy rocks.
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