sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale. Color: Grey, black, or brown. Luster: Waxy to dull. Crystal Structure: Trigonal (microcrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Grey, black, or brown
Luster: Waxy to dull

Identified May 27, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale. Color: Grey, black, or brown. Luster: Waxy to dull. Crystal Structure: Trigonal (microcrystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.

Formation & geological history

Formed by the precipitation of silica-rich fluids within sedimentary rock layers, often replacing limestone or capturing marine fossils. It can be biochemical (from diatom/radiolarian shells) or chemical in origin.

Uses & applications

Historically used for making stone tools (arrowheads, knives) and strike-a-lights. Today used as a decorative stone, in jewelry (as a gemstone), and occasionally as a source of silica for industrial purposes.

Geological facts

Chert has been used by humans for over 2 million years to create sharp edges. It is so hard that it can scratch steel and is incredibly resistant to chemical weathering.

Field identification & locations

Identify by its characteristic conchoidal (shell-like) fracture, waxy luster, and its ability to scratch glass. Commonly found as nodules in limestone or chalk beds, or as rounded pebbles in riverbeds and beaches.