sedimentary

Chert / Flint Pebble

Chert (SiO2)

Hardness: 7 on Mohs scale; Color: Brown, tan, or grayish; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline silicon dioxide; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 7 on Mohs scale
Color: Brown, tan, or grayish
Luster: Dull to waxy

Identified June 4, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of silica from groundwater filling pores in sedimentary rocks or from the accumulation of siliceous biological remains (like diatoms or radiolarians). Often found as water-worn pebbles in riverbeds or glacial deposits.

Uses & applications

Historically used for stone tools and starting fires; currently used in landscaping, as an abrasive, and sometimes tumbled for lapidary collections.

Geological facts

Chert was one of the most important minerals for early humans because its conchoidal fracture produces extremely sharp edges, ideal for arrowheads and scrapers. When struck against steel, it produces sparks.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, curved conchoidal fractures. Commonly found in gravel pits, river banks, and beaches.