Chalcedony (Common Opal variant)

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.

Identified on: 5/15/2026

Mode: Standard