sedimentary

Oolitic Chert

Ooidal Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

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

Identified April 6, 2026More sedimentary →

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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.