sedimentary
Oolitic Chert
Silicon Dioxide (SiO2) with Oolitic texture
Hardness: 6.5-7 (Mohs); Color: Tan, brown, or gray with concentric circular patterns; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- Tan, brown, or gray with concentric circular patterns
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Tan, brown, or gray with concentric circular patterns; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed by the precipitation of silica in marine environments, often replacing original oolitic limestone sequences. These small spherical grains (oids) form in warm, shallow, agitated waters during the Phanerozoic eon.
Uses & applications
Used primarily by lapidaries for making cabochons, jewelry, and decorative stones due to its unique pattern. Historically used for stone tools (arrowheads) because of its sharp conchoidal fracture.
Geological facts
The 'eyes' or circles seen in the rock are cross-sections of tiny spheres called ooids, which form like pearls by layers of mineral coating a grain of sand or shell fragment.
Field identification & locations
Identify in the field by searching for waxy-textured rocks with small, repetitive circular patterns. It will typically scratch glass and is commonly found in riverbeds or limestone outcrops in the Midwest USA.
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