sedimentary
Chert (often called Flint or Agate nodule)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown with a waxy luster; Crystal Structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or brown with a waxy luster
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown with a waxy luster; Crystal Structure: Microcrystalline/cryptocrystalline; 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 chemical precipitation in marine sedimentary environments. This specific nodule likely formed by silica replacing limestone or chalk.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Currently used in road construction (aggregate) and occasionally as a low-cost gemstone or decorative stone.
Geological facts
Chert nodules are incredibly durable and often survive long after the surrounding limestone bedrock has weathered away. It is spark-resistant unless struck with steel, which made it the primary material for flintlock firearms.
Field identification & locations
Identify in the field by its rounded, nodular shape, smooth waxy interior when broken, and its ability to scratch glass. Found globally in limestone and chalk deposits.
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