sedimentary

Chert (Vuggy or Fossilliferous)

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 Mohs scale. Color: Tan, beige, light brown. Luster: Dull to waxy. Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.65. Features numerous vugs (cavities) and a pitted texture.

Hardness: 7 Mohs scale
Color: Tan, beige, light brown
Luster: Dull to waxy

Identified July 13, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily through the chemical precipitation of silica or the replacement of limestone/chalk by silica-rich fluids. This specimen shows a 'vuggy' texture often resulting from the weathering out of fossils or carbonate minerals. Commonly Paleozoic to Mesozoic in age.

Uses & applications

Used historically for stone tools due to sharp edges; currently used in construction aggregate and as a mineral specimen for collectors.

Geological facts

Chert is extremely durable and chemically resistant. It often contains micro-fossils like radiolarians or diatoms. Some vuggy cherts are colloquially called 'morthead' rocks or 'sponge' rocks due to their appearance.

Field identification & locations

Identify in the field by its hardness (will scratch glass and steel), conchoidal fracture if broken, and lack of reaction to hydrochloric acid (unlike limestone). Commonly found in riverbeds and limestone outcrops.