sedimentary

Chert

Chert (Microcrystalline Quartz, SiO2)

Hardness: 7 (Mohs scale), Color: tan, cream, or light grey, Luster: waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Identified June 17, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale), Color: tan, cream, or light grey, Luster: waxy to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Formation & geological history

Formed by the precipitation of silica from groundwater or the accumulation of silica-rich microfossils (like radiolarians or diatoms) in marine environments. Often found as nodules in limestone or as bedded deposits.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used in construction aggregate and occasionally as a gemstone or for lighting fires (flint).

Geological facts

Chert has been used by humans for over 2 million years. Flint is a specific dark variety of chert. When struck against steel, chert can produce sparks, making it essential for early fire-starting tools.

Field identification & locations

Identify by its hardness (scratches glass), waxy appearance, and conchoidal (shell-like) fracture patterns. Commonly found in sedimentary basins and as river pebbles.