sedimentary

Chert (Chert Nodule with Cortex)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Tan, beige, and gray with off-white chalky rind; Luster: Dull to waxy; Structure: Cryptocrystalline/Massive; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Tan, beige, and gray with off-white chalky rind
Luster: Dull to waxy

Identified March 18, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like diatoms or sponges) in sedimentary basins, often replacing limestone or chalk. Age can range from Precambrian to Holocene.

Uses & applications

Historically used for stone tools (flintknapping) due to its sharp edges; industrially used as an abrasive, and sometimes in landscaping or as semi-precious jewelry (jasper/agate varieties).

Geological facts

This specimen features a distinct 'cortex'—the weathered, chalky outer layer. The circular dark feature is likely a cross-section of a fossilized sponge or a smaller nodule core. Chert is so durable it often survives long after the surrounding limestone bedrock has eroded away.

Field identification & locations

Identify by its extreme hardness (will scratch glass and steel), conchoidal (shell-like) fracture patterns, and smooth, waxy interior texture. Commonly found in limestone outcrops, riverbeds, and glacial till.

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