sedimentary
Chert (Common Opal/Jasper transition)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs); Color: Tan, cream, yellow-orange; Luster: Waxy to vitreous; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65
- Hardness
- 6
- Color
- Tan, cream, yellow-orange
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Tan, cream, yellow-orange; Luster: Waxy to vitreous; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65
Formation & geological history
Formed through the accumulation of silica-rich fluids or biological remains (like radiolarians) that precipitate in sedimentary basins. These specimens appear to be secondary silica replacements in a volcanic or sedimentary environment.
Uses & applications
Used primarily for lapidary work (cabochons), flintknapping (arrowheads), and as decorative landscaping or collector specimens. Historically used for making fire and tools.
Geological facts
Chert is so durable that it is often found as rounded pebbles in riverbeds far from its source. High-grade translucent varieties are often classified by collectors as 'Common Opal' or 'Chalcedony'.
Field identification & locations
Identify by its waxy texture and sharp, shell-like (conchoidal) fractures. It will scratch glass but cannot be scratched by a steel knife. Common in limestone regions and volcanic ash beds.
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