sedimentary

Chert (Flint variety)

Microcrystalline or cryptocrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, tan, or brown with a waxy to dull luster; Crystal structure: Trigonal (microscopic fibers); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Gray, tan, or brown with a waxy to dull luster

Identified March 24, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) on the ocean floor, or via the replacement of limestone by silica-rich groundwater. Often found in nodules or layers within sedimentary rock sequences.

Uses & applications

Historically used for stone tools (arrowheads/scrapers) and fire-starting. Currently used as road aggregate, in jewelry as semi-precious gemstones (jaspers/agates), and as high-pressure grinding media in industry.

Geological facts

Chert was one of the most important raw materials for early humans because its conchoidal fracture produces edges sharper than a steel scalpel. The dark variety is commonly known as flint.

Field identification & locations

Identify in the field by its smooth, waxy feel and shell-like (conchoidal) curved breaks. It will easily scratch glass. Found globally in limestone formations and riverbeds/beaches as water-worn pebbles.

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Brecciated Chert (Microcrystalline Silica)

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