sedimentary

Oolitic Chert

Silicon Dioxide (SiO2) with oolitic texture

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Tan, yellowish-brown, and white ooids; Luster: Waxy to vitreous; Structure: Microcrystalline with spherical ooids; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Tan, yellowish-brown, and white ooids
Luster: Waxy to vitreous

Identified June 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the replacement of oolitic limestone by silica. The original calcium carbonate ooids, which formed in warm, shallow marine environments, are replaced by chalcedony or microcrystalline quartz while preserving the concentric spherical structures.

Uses & applications

Primarily used as a lapidary material for polishing into cabochons, as a decorative specimen for collectors, and historically by indigenous peoples for making sharp tools and arrowheads due to its conchoidal fracture.

Geological facts

The spherical 'eyes' or ooids are actually concentric layers of material that grew around a nucleus, such as a grain of sand or a shell fragment, in agitated water before lithification and silica replacement occurred.

Field identification & locations

Identify in the field by looking for small, egg-like concentric circles (ooids) embedded in a dense, hard matrix that cannot be scratched by a steel nail. Commonly found in areas with ancient limestone beds like the Midwestern United States (e.g., Missouri, Illinois).