sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, black, or tan; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.

Hardness: 7 on Mohs scale
Color: Gray, black, or tan
Luster: Waxy to dull

Identified June 29, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, black, or tan; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.

Formation & geological history

Formed in marine environments from the accumulation of siliceous remains of microscopic organisms like diatoms or radiolarians, which dissolve and reprecipitate as microcrystalline silica within limestone or shale beds.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when fractured. Modern uses include road surfacing, decorative garden stone, and occasionally in jewelry as chalcedony.

Geological facts

Chert is so hard that it can strike sparks against steel, a property that was used in flintlock firearms. When it is high-quality and brightly colored, it is often called Jasper.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It is commonly found as nodules in limestone or as bedded deposits in riverbeds.