sedimentary
Chert (including Common Opal and Chalcedony)
Silicon Dioxide (SiO2 · nH2O)
Hardness: 6.5-7 (Mohs scale); Color: Variegated including white, cream, and deep teal/green; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.1-2.6
- Hardness
- 6
- Color
- Variegated including white, cream, and deep teal/green
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Variegated including white, cream, and deep teal/green; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.1-2.6
Formation & geological history
Formed through the accumulation of siliceous remains of organisms or precipitation from silica-rich fluids in sedimentary basins or volcanic cavities. Often found in nodules within limestone or as bedded deposits.
Uses & applications
Historical use for stone tools/edges; modern use in lapidary arts, jewelry (cabochons), and as aquarium or decorative stones.
Geological facts
This specimen shows a beautiful transition between opaque chert and translucent common opal or chalcedony. The smooth, curved 'scoops' on the surface are conchoidal fractures, a key identifier of silica-rich minerals.
Field identification & locations
Identify in the field by its hardness (scratches glass), waxy luster, and conchoidal 'shell-like' fractures. Commonly found in riverbeds, desert pavements, and exposed sedimentary strata across North America and Europe.
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