sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 Mohs; Color: Brown, tan, ochre; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65

Hardness: 6
Color: Brown, tan, ochre
Luster: Dull to waxy

Identified July 18, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Brown, tan, ochre; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65

Formation & geological history

Formed from the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation in marine or groundwater environments. This specimen likely weathered from a larger nodule in limestone or dolomite.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges when broken. Today used as road gravel, construction aggregate, and occasionally for lapidary work.

Geological facts

Chert is so durable that it often survives as pebbles in riverbeds and on beaches long after the surrounding limestone host rock has eroded away. It produces sparks when struck against steel.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and waxy, smooth texture compared to surrounding rocks. Often found as rounded pebbles on shorelines.