sedimentary

Chert (Grey/Black variety)

Chert (Silicon Dioxide, SiO2)

Hardness: 7 on Mohs scale; Color: Dark grey to charcoal; Luster: Waxy to vitreous (especially when polished); Crystal structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Dark grey to charcoal
Luster: Waxy to vitreous (especially when polished)

Identified March 17, 2026More sedimentary →

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Physical properties

Formation & geological history

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

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used as high-quality road surfacing, in landscaping, and as a semi-precious stone for tumbling and jewelry making.

Geological facts

Because of its extreme hardness and ability to hold a razor-sharp edge, chert was one of the most vital survival resources for prehistoric humans. It is chemically identical to flint, but 'flint' is often specifically used to describe chert found in chalk deposits.

Field identification & locations

Identify in the field by its smooth, waxy surface and conchoidal (shell-like) fracture patterns. It will scratch glass easily. Commonly found in riverbeds, glacial till, and embedded in limestone outcrops globally.

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