sedimentary
Oolitic Chert
Silicon Dioxide (SiO2) with oolitic texture
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
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Physical properties
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.
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).
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