sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 (Mohs scale), Color: light grey with iron staining, Luster: waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Hardness: 6

Identified June 5, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs scale), Color: light grey with iron staining, Luster: waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Formation & geological history

Formed primarily in marine environments where silica-rich fluids replace calcium carbonate in limestone or chalk beds, or through the accumulation of siliceous marine organisms like diatoms and radiolaria.

Uses & applications

Historically used for stone tools and starting fires; currently used as construction aggregate, in road surfacing, and as a durable lapidary material for tumbling.

Geological facts

Chert is so hard it will scratch glass. Because of its conchoidal fracture (breaking like glass with curved edges), it was one of the primary materials used by prehistoric humans to create arrowheads and knives.

Field identification & locations

Identified in the field by its hardness, smooth waxy texture, and sharp, curved edges when broken. It is extremely common in riverbeds and limestone outcrops globally.