mineral
Chalcedony (Common Opal variant)
Silicon dioxide (SiO2) with microcrystalline structure
Hardness: 6.5-7 on Mohs scale; Color: Milky white to pale yellow/honey; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None; Specific Gravity: 2.58-2.64
- Hardness
- 6
- Color
- Milky white to pale yellow/honey
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Milky white to pale yellow/honey; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None; Specific Gravity: 2.58-2.64
Formation & geological history
Formed from the deposition of silica-rich water in cavities of volcanic and sedimentary rocks at low temperatures. Common in various geological ages ranging from Cenozoic to ancient Precambrian deposits.
Uses & applications
Used in gemstone carvings, jewelry making, lapidary arts, and as a collector's specimen. Historically used for making tools and seals.
Geological facts
Chalcedony was highly prized in antiquity; the name is derived from the ancient Greek town of Chalkedon. It is a fibrous cryptocrystalline form of silica.
Field identification & locations
Identify in the field by its waxy luster, conchoidal fracture (curved glass-like breaks), and lack of visible crystals. Commonly found in desert environments, riverbeds, and volcanic geodes.
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