sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on the Mohs scale; Color: Varies from white, grey, light blue, to brown and black; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline (quartz); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.60 to 2.65

Hardness: 7 on the Mohs scale
Color: Varies from white, grey, light blue, to brown and black
Luster: Waxy to dull

Identified July 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the chemical precipitation of silica from seawater, often replacing carbonate rocks like limestone during diagenesis. Commonly found in sedimentary formations worldwide.

Uses & applications

Historically used by early humans for stone tools, arrowheads, and fire-starting due to its sharp conchoidal fracture. Today used in abrasives, road construction, and decorative landscaping.

Geological facts

Chert that is dark grey or black is often specifically called flint. When struck against steel, chert produces sparks, making it crucial for historical fire-making kits.

Field identification & locations

Easily identified by its conchoidal (shell-like) fracture, waxy luster, and high hardness that scratches glass. Commonly found as nodules or layers within chalk and limestone formations.