sedimentary

Chert (specifically Flint)

Silicon Dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Gray to tan internal, white/brown exterior cortex; Luster: Waxy to vitreous; Structure: Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Gray to tan internal, white/brown exterior cortex
Luster: Waxy to vitreous

Identified February 19, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica from seawater or groundwater, often replacing organic matter or calcium carbonate in limestone/chalk beds. Commonly dates from the Cretaceous period.

Uses & applications

Historically used for stone tools (arrowheads, knives) and flintlock firearms. Currently used for road gravel, concrete aggregate, and occasional gemstone cabochons.

Geological facts

Due to its conchoidal fracture, it breaks into razor-sharp edges. It was arguably the most important technological material for early humans. It can create sparks when struck against steel.

Field identification & locations

Identifiable by its smooth, shell-like (conchoidal) fracture surfaces and waxy texture. Frequently found as nodules in chalk or limestone cliffs and along riverbeds. Look for a 'rind' or cortex on the outside.