sedimentary
Chert (Quartz Variety)
Microcrystalline Silica (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Tan, beige, brown, or light grey with pinkish banding; Luster: Dull to waxy; Structure: Cryptocrystalline (microscopic crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.
- Hardness
- 6
- Color
- Tan, beige, brown, or light grey with pinkish banding
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Tan, beige, brown, or light grey with pinkish banding; Luster: Dull to waxy; Structure: Cryptocrystalline (microscopic crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or by chemical precipitation from groundwater within limestone or chalk beds. Often occurring as nodules or beds in sedimentary layers from various geological ages, notably the Paleozoic and Mesozoic eras.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges; modernly used as construction aggregate, road base, and occasionally as decorative landscaping stone or for primitive fire-starting (flint).
Geological facts
Chert is so durable that it often remains in riverbeds long after the surrounding limestone has eroded away. The dark variety of chert is most commonly known as 'flint.' It was the primary technological material for humanity for over two million years.
Field identification & locations
Identify in the field by its smooth, waxy texture and distinctive 'shell-like' curved (conchoidal) fractures. It will easily scratch glass but cannot be scratched by a steel knife. Commonly found in riverbeds, gravel pits, and eroding limestone outcrops.
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