sedimentary

Chert (containing Fossil Inclusions)

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Gray, tan, or black with lighter inclusions; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray, tan, or black with lighter inclusions
Luster: Waxy to dull

Identified March 20, 2026More sedimentary →

Explore Chert (containing Fossil Inclusions) in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 7 on Mohs scale; Color: Gray, tan, or black with lighter inclusions; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Formation & geological history

Formed from the precipitation of silica-rich fluids in limestone or chalk beds, often replacing organic material. These specimens typically range from the Paleozoic to Mesozoic eras (approx. 66 to 500 million years old).

Uses & applications

Historically used for stone tools (flintknapping) due to its sharp edges. Modernly used as road aggregate, architectural stone, and for educational fossil collecting.

Geological facts

Chert nodules often trap ancient marine life like crinoids or brachiopods. Because silica is so durable, these fossils remain intact long after the surrounding limestone has eroded away.

Field identification & locations

Identify by its 'shell-like' conchoidal fracture and its ability to scratch glass. Commonly found in riverbeds or gravel pits in limestone regions like the American Midwest or the UK.