sedimentary

Chert (Field Stone)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Gray, brown, and tan tones with earthy weathering; Luster: Dull to waxy; Structure: Finely crystalline (not visible to naked eye); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.

Hardness: 6
Color: Gray, brown, and tan tones with earthy weathering
Luster: Dull to waxy

Identified June 17, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from groundwater within other sedimentary rock layers such as limestone. These specimens can range from hundreds of millions of years old to relatively recent.

Uses & applications

Used historically for stone tools (arrowheads, scrapers) due to its sharp fracture edges. Locally used today as construction aggregate, road base, or decorative garden stone.

Geological facts

Chert is so durable that it often remains as 'river gravel' or 'field stones' long after the surrounding softer rocks have eroded away. It was a primary material for prehistoric humans across the globe.

Field identification & locations

Identify by its hardness (it will scratch glass) and its shell-like (conchoidal) fracture when broken. Frequently found in stream beds, fields, and outcrops of limestone or dolomite.

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