sedimentary

Chert or flint fragment

Microcrystalline quartz (SiO₂), commonly called chalcedony or flint

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; brown to gray, translucent at thin edges, with a waxy to dull luster and conchoidal fracture. It lacks visible grains and cleavage; specific gravity is about 2.6.

Identified August 8, 2026More sedimentary →

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Physical properties

Hardness 6.5–7 Mohs; brown to gray, translucent at thin edges, with a waxy to dull luster and conchoidal fracture. It lacks visible grains and cleavage; specific gravity is about 2.6.

Formation & geological history

Forms when silica-rich groundwater replaces or fills pores in limestone, chalk, or sediments, often during early diagenesis. Its precise geological age cannot be determined from the image; chert occurs from Precambrian rocks through modern deposits.

Uses & applications

Used historically for sharp tools and fire-starting, and today as decorative stone, lapidary material, and occasionally aggregate. Attractive translucent pieces may be tumbled or cut.

Geological facts

The sharp curved fracture surfaces are characteristic of cryptocrystalline quartz. The reddish-brown coloration likely comes from iron oxides or other impurities.

Field identification & locations

Look for glass-like conchoidal fractures, very high hardness, and inability to scratch it with a steel knife; it should scratch ordinary glass. Common in limestone, chalk, river gravels, and glacial sediments worldwide.