sedimentary

Chert (Chert Nodule with Cortex)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Tan, brown, and gray; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Tan, brown, and gray
Luster: Waxy to dull

Identified March 20, 2026More sedimentary →

Explore Chert (Chert Nodule with Cortex) 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: Tan, brown, and gray; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & geological history

Formed through the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like radiolarians or diatoms) in limestone or chalk beds. Often found as nodules within marine sedimentary layers.

Uses & applications

Historically used for stone tools (knapping) due to its sharp edges; modernly used in road construction as aggregate, and sometimes as a decorative stone in landscaping or jewelry.

Geological facts

Chert has been used by humans for over 2 million years to create tools. Flint is a specific dark-colored variety of chert often associated with chalk deposits. The white outer layer seen in the image is the 'cortex,' the unweathered interface between the silica and the host rock.

Field identification & locations

Identify in the field by its waxy interior and shell-like (conchoidal) fracture when broken. It cannot be scratched by a steel knife. Commonly found in riverbeds or eroded limestone outcrops.