sedimentary

Chert (Fossiliferous)

Chert (Cryptocrystalline Silica/SiO2)

AI-generated identification · may be incorrect

Hardness: 7 (Mohs scale), Color: Gray with tan inclusions, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Identified June 12, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale), Color: Gray with tan inclusions, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Formation & geological history

Formed through the accumulation of silica-rich marine organisms or chemical precipitation within limestone/chalk beds, often during the Paleozoic or Mesozoic eras. The weathering of softer host rock leaves behind these durable nodules.

Uses & applications

Historically used for stone tools/arrowheads. Modern uses include decorative landscaping, road base, and occasionally as semi-precious gemstones when displaying unique patterns.

Geological facts

This specimen appears to be a water-worn nodule containing trace fossils or vugs where fossils were once present. Chert is so hard it can strike sparks from steel, a property used by early humans for fire-starting.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), waxy feel, and smooth, rounded edges if water-worn. Found globally in areas with sedimentary basin legacies, especially riverbeds and shorelines.