sedimentary

Chert (often called Flint or Agate nodule)

Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

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

Identified April 13, 2026More sedimentary →

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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.