sedimentary

Chert / Flint

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, beige, or light brown (due to iron oxide impurities); Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Tan, beige, or light brown (due to iron oxide impurities)
Luster: Waxy to dull

Identified March 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the chemical replacement of limestone/chalk by silica-rich fluids. Often found as nodules in limestone beds; ages vary from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools and weapons (arrowheads) due to its sharp edges. Modernly used in road construction, as jewelry (banded varieties), and in flint-and-steel fire starters.

Geological facts

Chert has a unique property called conchoidal fracture, which allows it to break into sharp, curved surfaces similar to glass. It was one of the most vital resources for prehistoric humans globally.

Field identification & locations

Identify in the field by its hardness (it will easily scratch glass) and its curved, shell-like fracture pattern. It feels smooth/waxy and is often found as rounded nodules in light-colored sedimentary rock like limestone.