sedimentary

Chert (Quartz Variety)

Chert (SiO2)

Hardness: 7 (Mohs scale); Color: Tan, beige, or yellowish-brown; Luster: Dull to waxy; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 7 (Mohs scale)
Color: Tan, beige, or yellowish-brown
Luster: Dull to waxy

Identified March 4, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Tan, beige, or yellowish-brown; Luster: Dull to waxy; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from groundwater within limestone or chalk beds. Most specimens date from the Paleozoic to Mesozoic eras.

Uses & applications

Historically used for stone tools and arrowheads due to its sharp edges when broken. Modern uses include construction aggregate, road surfacing, and as a decorative stone in landscaping.

Geological facts

Chert is so durable that it often survives as smooth river pebbles long after the surrounding limestone has eroded away. It is closely related to flint, jasper, and agate.

Field identification & locations

Identify by its extreme hardness (will scratch glass and steel) and smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds, glacial till, and coastal areas throughout North America and Europe.