sedimentary

Chert

Chert (Microcrystalline Quartz, SiO2)

Hardness: 7 Mohs, Color: Gray, tan, white, and black mottled, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Identified June 9, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs, Color: Gray, tan, white, and black mottled, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Formation & geological history

Formed primarily through the chemical precipitation of silica from seawater or the accumulation of silica-rich biogenic remains like sponge spicules. Most commonly found in carbonate rock formations as nodules or layered beds.

Uses & applications

Used historically for stone tools due to its sharp edges; currently used as construction aggregate, in road surfacing, or occasionally for decorative landscaping.

Geological facts

Because it is so hard and breaks with a conchoidal fracture (like glass), it was one of the most important materials for early humans to make arrowheads and scrapers. Flint is a variety of chert.

Field identification & locations

Identify by its hardness (it will scratch glass), waxy feel, and smooth curved fracture surfaces. Found worldwide in limestone and dolostone outcrops.