sedimentary

Chert (also known as Flint or Common Chalcedony)

Cryptocrystalline Silica (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Tan, brown, white, and translucent grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.6 – 2.7.

Hardness: 7 on Mohs scale
Color: Tan, brown, white, and translucent grey
Luster: Waxy to dull

Identified March 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation and chemical precipitation of silica-rich fluids within sedimentary rock layers, often replacing carbonate minerals or biological remains like shells and fossils. It can form as nodules or bedded layers in limestone or chalk through geological ages (Proterozoic to modern).

Uses & applications

Historically used for stone tools and arrowheads; used in construction for road base and concrete aggregate. High-quality specimens are used in jewelry (cabochons) or as decorative lapidary material.

Geological facts

Because it breaks with a conchoidal fracture (smooth, curved surfaces), flint was the primary resource for Stone Age technology. It is often found as smooth nodules inside chalk beds. It is chemically very resistant and often remains on the surface long after the surrounding limestone has weathered away.

Field identification & locations

Identify in the field by its high hardness (it will scratch glass and steel), waxy luster, and smooth, curved 'shell-like' chip marks (conchoidal fracture). It is common in riverbeds, glacial deposits, and sedimentary outcrops worldwide.