sedimentary

Black Chert (Flint / Basanite)

Microcrystalline / Cryptocrystalline Quartz (SiO2)

Hardness 7 on Mohs scale; jet black to dark grey color; vitreous to waxy luster; amorphous/cryptocrystalline micro-structure; distinct shell-like conchoidal fracture with sharp edges; specific gravity 2.65.

Identified July 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through sedimentary chemical or biochemical processes where silica accumulates from diatoms or radiolarians, undergoing diagenesis and recrystallization over millions of years within limestone or chalk beds.

Uses & applications

Historically used for making sharp tools like arrowheads, scrapers, and flints for fire-starting. Today used in lapidary work, jewelry, landscaping, aggregate, and touchstones for testing precious metals.

Geological facts

Black chert with microcrystalline quartz texture is very durable and sharp when fractured. High-quality smooth black chert was historically used as a 'touchstone' to test the purity of gold and silver by the streak left on its surface.

Field identification & locations

Look for a smooth, glass-like texture with conchoidal (curved) fracture surfaces, extreme hardness (scratches glass easily), and an opaque deep dark tone. Commonly found in limestone, dolomite, or chalk deposits globally.

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