sedimentary

Chert

Chert (Microcrystalline Quartz, SiO2)

Hardness: 7 Mohs scale; Color: Brown, tan, cream; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 Mohs scale
Color: Brown, tan, cream
Luster: Waxy to dull

Identified July 9, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs scale; Color: Brown, tan, cream; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed from the accumulation of siliceous remains of microscopic marine organisms (like diatoms or radiolarians) or through chemical precipitation from silica-rich fluids in limestone/dolomite cavities. It can range from Precambrian to modern ages.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp conchoidal fracture. Modernly used as road aggregate, in landscaping, and some decorative varieties as semi-precious gemstones.

Geological facts

Chert is so hard it can spark when struck against steel, which made it essential for flintlock firearms and early fire-starting kits. The variety known as 'flint' is a sub-type of chert.

Field identification & locations

Identified by its extreme hardness (scratches glass), waxy appearance, and shell-like (conchoidal) fracture surfaces. Commonly found as nodules or layers within limestone beds.