sedimentary

Chert Pebble

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, tan, and dark charcoal; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.

Hardness: 7 on Mohs scale
Color: Gray, tan, and dark charcoal
Luster: Waxy to dull

Identified June 15, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, tan, and dark charcoal; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from groundwater within limestone or chalk beds. This specimen is water-worn into a river cobble shape.

Uses & applications

Historically used for making stone tools (arrowheads). Modernly used as decorative landscaping stone, in construction aggregate, or for flint-striking to start fires.

Geological facts

Chert is so hard it can scratch glass and steel. Darker varieties are often called flint. It is one of the most durable rocks, often surviving long-term river transport as rounded pebbles.

Field identification & locations

Identify in the field by its extreme hardness, smooth waxy texture when wet, and distinctive shell-like (conchoidal) fractures. Found globally in riverbeds and coastal areas.