sedimentary

Chert (and Chalcedony/Agate mixture)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, and white with translucent windows; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65.

Hardness: 6
Color: Gray, tan, and white with translucent windows
Luster: Waxy to vitreous

Identified March 24, 2026More sedimentary →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed through the accumulation of silica-rich fluids in limestone cavities or as deep-sea biochemical deposits. The specimen shows rounding from alluvial transport (river action) and may date back anywhere from 10 to 500 million years depending on the region.

Uses & applications

Used historically for stone tools and arrowheads; currently used as landscaping gravel, in concrete aggregate, and as a semi-precious gemstone when polished into agates.

Geological facts

Chert was the most important industrial mineral of the Stone Age due to its ability to hold a razor-sharp edge. It is virtually immune to chemical weathering, which is why it often appears as rounded pebbles in stream beds while other rocks disintegrate.

Field identification & locations

Identify in the field by its hardness (it will scratch glass and steel) and its shell-like (conchoidal) fracture pattern. Commonly found in river beds, beaches, and glacial till. Look for translucent edges when held to light.

Other sedimentary specimens

See all

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