sedimentary

Chert (Vuggy or Fossilliferous)

Microcrystalline Quartz (SiO2)

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 →

Explore Chert (Vuggy or Fossilliferous) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

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.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sedimentary Sandstone

Arenite (Sedimentary Rock)

sedimentary

Fossiliferous Concretion

Septarian Concretion containing Brachiopods/Crinoids

fossil

Shale

Shale (clastic sedimentary rock)

sedimentary

Shale

Argillaceous sedimentary rock

sedimentary

Mudstone

Argillite / Siliciclastic sedimentary rock

sedimentary