sedimentary
Chert (Chert Nodule with Cortex)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
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 More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
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.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone (Quartz-rich)
Arenite (SiO2)
sedimentary
Sandstone (with man-made markings)
Arenite (composed primarily of Quartz, SiO2)
sedimentary