sedimentary

Chert (Jasper variety)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Dark brown to black with reddish-brown inclusions; Luster: Dull to waxy; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.58–2.91

Hardness: 6
Color: Dark brown to black with reddish-brown inclusions
Luster: Dull to waxy

Identified March 25, 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 from the accumulation of silica-rich microfossils (like radiolarians or diatoms) or chemical precipitation on the ocean floor. Most specimens found in stream beds represent erosion from Mesozoic or Paleozoic sedimentary layers.

Uses & applications

Historically used for stone tools and arrowheads due to its sharp conchoidal fracture. Modern uses include decorative gravel, lapidary work (jewelry cabochons), and as a historical/teaching specimen.

Geological facts

This specific piece is a water-worn river pebble. The lighter tan/brown markings are likely iron oxide 'rind' or inclusions. Chert is so durable that it can survive millions of years of tumbling in river systems without disintegrating.

Field identification & locations

Identify in the field by its smooth, waxy texture and inability to be scratched by a steel knife. It often occurs as nodules in limestone. Collectors should look for unique patterns or 'pictures' within the stone, which can increase value.