sedimentary

Chert (Flint)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 Mohs; Color: Dark grey to black; Luster: Waxy to dull; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 6
Color: Dark grey to black
Luster: Waxy to dull

Identified July 15, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Dark grey to black; Luster: Waxy to dull; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & geological history

Formed in marine environments through the precipitation of silica from organic remains (like diatoms or radiolarians) or chemical replacement of carbonate rocks. Often found as nodules in limestone or chalk beds.

Uses & applications

Historically used for stone tools (arrowheads, knives) and strike-a-lights for fire starting. Modernly used as a decorative stone, in construction aggregate, and by flintknapping hobbyists.

Geological facts

Flint is a variety of chert. Due to its conchoidal fracture, it produces edges sharper than a steel scalpel, leading to its extensive use in the Stone Age.

Field identification & locations

Identify by its waxy texture, extreme hardness (it will scratch glass), and its tendency to break into sharp-edged flakes. Commonly found in riverbeds or coastal cliffs where limestone has eroded.