sedimentary

Chert (Iron-stained)

Chert (SiO2)

Hardness: 7 (Mohs scale), Color: tan, grey, brown due to iron oxides, Luster: dull to waxy, Crystal Structure: Microcrystalline quartz, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Identified June 21, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale), Color: tan, grey, brown due to iron oxides, Luster: dull to waxy, Crystal Structure: Microcrystalline quartz, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Formation & geological history

Formed from the accumulation of silica-rich microfossils or through chemical precipitation in underwater environments. Manganese or iron oxides often coat or permeate the specimen creating dark patches.

Uses & applications

Historically used for stone tools (flintknapping); currently used in road construction, landscaping, and as an industrial abrasive.

Geological facts

Chert is a tough, durable rock that was the primary material for spearheads and arrowheads for thousands of years due to its predictable conchoidal fracturing.

Field identification & locations

Look for sharp glass-like edges where the rock has fractured and a waxy texture. It is extremely hard and will scratch glass easily. Commonly found in limestone or chalk beds.