mineral

Chalcedony (Common Opal or Agate)

Silicon Dioxide (SiO2)

Hardness: 6.5-7.0 (Mohs scale); Color: Waxy blue-grey with yellow-orange inclusions; Luster: Vitreous to waxy; Crystal structure: Cryptocrystalline; Cleavage: None; Specific gravity: 2.58-2.64

Hardness: 6
Color: Waxy blue-grey with yellow-orange inclusions
Luster: Vitreous to waxy

Identified May 26, 2026More mineral →

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Physical properties

Hardness: 6.5-7.0 (Mohs scale); Color: Waxy blue-grey with yellow-orange inclusions; Luster: Vitreous to waxy; Crystal structure: Cryptocrystalline; Cleavage: None; Specific gravity: 2.58-2.64

Formation & geological history

Forms at low temperatures by the precipitation of silica from groundwater in the cavities of volcanic or sedimentary rocks, often associated with hydrothermal activity.

Uses & applications

Used primarily in lapidary work, jewelry (beads and cabochons), carvings, and as a popular collector's mineral.

Geological facts

Chalcedony was prized in the ancient world for making cylinder seals and jewelry because it does not stick to hot wax. This specimen shows 'dendritic' or 'moss' like staining from iron or manganese impurities.

Field identification & locations

Identify in the field by its waxy luster, conchoidal fracture (smooth, curved breaks), and hardness which will scratch glass but cannot be scratched by a steel knife. Frequently found in riverbeds or desert regions.

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