sedimentary
Oolitic Chert
Oolitic Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs); Color: White, gray, tan with black/dark ooids; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- White, gray, tan with black/dark ooids
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs); Color: White, gray, tan with black/dark ooids; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the replacement of oolitic limestone by silica-rich fluids. The original ooids (small, spherical sedimentary grains) are preserved in the silica matrix. These often date back to the Paleozoic era in shallow marine environments.
Uses & applications
Used in tool-making historically (knapping), as decorative landscaping aggregate, and for lapidary work including cabochons for jewelry. It is also used as a road base material.
Geological facts
The term 'ooid' comes from the Greek word 'oion' for egg. These specimens are essentially fossilized sand dunes or seabed ripples where calcium carbonate layers built up around a nucleus before being turned into hard chert.
Field identification & locations
Identify in the field by its hardness (will scratch glass), conchoidal fracture patterns, and distinctive 'egg-like' or 'polka-dot' circular patterns. Found commonly in the Midwest USA and river beds throughout the Ozarks.
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