mineral

Chalcedony (Common Opal/Chert variety)

Silicon Dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 (Mohs); Color: Waxy white to translucent gray; Luster: Waxy to vitreous; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64

Hardness: 6
Color: Waxy white to translucent gray
Luster: Waxy to vitreous

Identified March 25, 2026More mineral →

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Physical properties

Formation & geological history

Formed from the deposition of silica-rich fluids in cavities or fractures within volcanic or sedimentary rocks. It often precipitates at low temperatures over millions of years as groundwater filters through volcanic ash or decomposing silica-heavy rocks.

Uses & applications

Used primarily in lapidary work for cabochons and carvings, as an abrasive in some industrial processes, and as a component in high-quality ceramics. Historical use includes the making of stone tools due to its sharp conchoidal fracture.

Geological facts

Chalcedony is a cryptocrystalline form of silica, meaning its crystals are so small they can only be seen under extreme magnification. It has been used for over 3,000 years to make seals, jewelry, and beads in many Mediterranean and Middle Eastern cultures.

Field identification & locations

Identify in the field by its waxy appearance and smooth, shell-like (conchoidal) breakage patterns. It will scratch glass but cannot be scratched by a steel knife. Common in volcanic regions and sedimentary nodules worldwide.