sedimentary
Oolitic Chert
Ooidal Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, or grayish; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, or grayish
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, or grayish; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the replacement of calcium carbonate ooids (small round sedimentary grains) by silica in marine environments. Most common in Paleozoic limestone layers where groundwater rich in silica has altered the original rock.
Uses & applications
Used historically for stone tools due to conchoidal fracture; modernly used as decorative landscape stone, in jewelry as cabochons, and as a geological study specimen.
Geological facts
The term 'oolitic' comes from the Greek word 'oon' for egg, because the small rounded grains look like fish roe. When these grains are replaced by silica, the result is a rock harder than steel.
Field identification & locations
Identify in the field by looking for a 'bumpy' or 'fish egg' texture on weathered surfaces and a smooth, glass-like conchoidal fracture when broken. Frequently found in riverbeds in the Midwestern United States or near limestone outcrops.
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