sedimentary

Chert (Black Chert/Flint)

Cryptocrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Black to dark gray with occasional tan/brown surface staining; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline (microscopic crystals); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 (Mohs scale)
Luster: Waxy to vitreous

Identified February 17, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation within limestone or chalk beds. Most specimens found in surface gravels date from the Paleozoic to Mesozoic eras.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) and strike-a-lights due to sparks; modernly used as road aggregate, architectural stone, and for lapidary tumbling.

Geological facts

Because it breaks with a conchoidal (shell-like) fracture, it creates edges sharper than a surgeon's scalpel. Black chert is often specifically referred to as 'Flint' when found in chalk deposits.

Field identification & locations

Identify in the field by its hardness (will scratch glass), waxy feel, and sharp, curved edges where it has broken. Look for it in river beds, gravel deposits, and limestone outcrops.

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