sedimentary

Flint

Silicon Dioxide (SiO2) - variety of Chert

Hardness: 7 Mohs; Color: Dark grey, black, or deep brown; Luster: Waxy or dull; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 Mohs
Color: Dark grey, black, or deep brown
Luster: Waxy or dull

Identified June 24, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Dark grey, black, or deep brown; Luster: Waxy or dull; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed as nodules in sedimentary rocks such as chalk or limestone. It occurs from the replacement of carbonate minerals by silica from dissolved sponge spicules or radiolarians, typically in marine environments during the Cretaceous period.

Uses & applications

Historically used for stone tools and weapons (knapping); used as a spark generator in flintlock firearms; currently used in glass manufacturing and as an abrasive.

Geological facts

Flint was one of the most important resources for prehistoric humans. When struck against steel, it produces a spark that can start a fire, due to small particles of iron being shaved off and oxidizing rapidly.

Field identification & locations

Identify by its characteristic conchoidal fracture (curved, shell-like breaks) and sharp edges. It is common in the chalk downs of England, France, and parts of the United States. Collectors look for pieces with significant cortex (outer white weathering) or worked edges.