Chert (Flint)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: grey, tan, brown, or white; Luster: waxy to dull; Crystal structure: microcrystalline/cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.5-2.8

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. Most specimens date from the Phanerozoic eon.

Uses & Applications

Used historically for stone tools and weapons (arrowheads); currently used in road construction, as a grinding medium in ball mills, and occasionally in jewelry as or 'river rock' landscaping.

Geological Facts

Chert is so hard it can scratch steel. When struck against steel, it produces sparks, which led to its widespread historical use in 'flintlock' firearms and fire-starting kits.

Field Identification & Locations

Identify by its 'shell-like' conchoidal fracture patterns and waxy texture. It is often found as nodules or layers within sedimentary rocks like limestone or dolomite across the globe.

Identified on: 4/26/2026

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