sedimentary

Chert (Flint)

Chert (Microcrystalline or Cryptocrystalline Quartz, SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Black, grey, or brown core with a tan/white weathering rind (cortex); Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).

Hardness: 7 on Mohs scale
Luster: Waxy to dull

Identified June 19, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Black, grey, or brown core with a tan/white weathering rind (cortex); Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).

Formation & geological history

Formed primarily through the chemical precipitation of silica-rich fluids or the accumulation of siliceous skeletal remains (like diatoms or radiolarians) within limestone or chalk beds. Often found in nodules or layers throughout various Phanerozoic geological periods.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp edges. Today used in construction aggregate, road surfacing, and as a component in jewelry or lapidary arts.

Geological facts

Flint was the primary material for the Stone Age technology. It sparks when struck against steel, making it vital for early fire-starting and flintlock firearms. The light-colored outer layer is called a 'cortex'.

Field identification & locations

Identified by its extremely smooth, glass-like 'conchoidal' fracture patterns and its ability to scratch glass. Commonly found in chalk or limestone outcrops, or as polished pebbles in riverbeds.