Agate (Chalcedony)

Physical Properties

Hardness: 6.5-7 on Mohs scale. Color: Translucent gray, white, and brown. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline (trigonal). Cleavage: None (conchoidal fracture). Specific gravity: 2.58-2.64.

Formation & Geological History

Formed primarily in volcanic and metamorphic rocks where silica-rich groundwater fills cavities (vesicles) or cracks. The silica precipitates over long periods, often in concentric layers.

Uses & Applications

Commonly used for lapidary arts, jewelry (cabochons), burnishing tools, precision pendulums, and as ornamental collector specimens.

Geological Facts

Agate is a variety of chalcedony. It was named by the Greek philosopher Theophrastus who discovered the stone along the shore of the Achates River in Sicily.

Field Identification & Locations

Identify in the field by its waxy appearance, conchoidal (shell-like) fracture, and translucent nature when held to light. Often found in riverbeds or areas with ancient volcanic activity.

Identified on: 4/19/2026

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