sedimentary

Chert (variations include Flint/Chalcedony)

Chert (SiO2 - Silicon Dioxide)

Hardness: 6.5-7 on Mohs scale. Color: Tan, brown, gray, or yellow. Luster: Waxy to dull. Crystal Structure: Microcrystalline/Cryptocrystalline (Trigonal system). Cleavage: None (Conchoidal fracture). Specific Gravity: 2.6-2.65.

Hardness: 6
Color: Tan, brown, gray, or yellow
Luster: Waxy to dull

Identified April 3, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolarians) or through the chemical precipitation of silica from groundwater within sedimentary layers like limestone or chalk.

Uses & applications

Abrasives, decorative landscaping, historical stone tools (arrowheads), and sometimes used in gemstone carving or as a road aggregate.

Geological facts

Chert was one of the most important minerals to prehistoric humans because it breaks with a conchoidal fracture, creating razor-sharp edges used for knives and scrapers. Some varieties, like flint, can produce sparks when struck against steel.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds, gravel pits, and eroding out of limestone cliffs.

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