sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs), Color: Tan to light brown (due to iron oxides), Luster: Dull to waxy, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Identified July 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in marine or freshwater environments as a chemical precipitate or through the replacement of limestone. It originates from the accumulation of silica-rich organic remains such as sponge spicules or radiolarians, often dating back hundreds of millions of years.

Uses & applications

Used historically for stone tools and arrowheads due to its sharp conchoidal fracture. Today used as road gravel, decorative garden stone, and as a raw material in some ceramic manufacturing.

Geological facts

Chert is so hard it can scratch steel and will produce sparks when struck against iron; this made it essential for early flintlock firearms. Flint is a high-quality, dark-colored variety of chert.

Field identification & locations

Identify in the field by its superior hardness, smooth waxy feel when freshly broken, and characteristic curved (half-moon) fracture patterns. Frequently found as nodules within limestone or as bedded deposits in sedimentary basins.