mineral

Chalcedony (specifically Agate or Common Opal variety)

Cryptocrystalline Silica (SiO2)

Hardness: 6.5 to 7 on Mohs scale. Color: Waxy yellow-orange to translucent tan. Luster: Vitreous to waxy. Crystal structure: Cryptocrystalline (hexagonal/trigonal). Cleavage: None, exhibits conchoidal fracture. Specific gravity: 2.60-2.64.

Hardness: 6
Color: Waxy yellow-orange to translucent tan
Luster: Vitreous to waxy

Identified January 24, 2026More mineral →

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Physical properties

Formation & geological history

Formation occurs at low temperatures via precipitation from silica-rich groundwater filling cavities or veins in volcanic or sedimentary rocks. It often forms as nodules or geodes over thousands to millions of years.

Uses & applications

Used extensively as a gemstone in jewelry (cabochons), as an industrial abrasive, and in the manufacturing of laboratory mortars and pestles due to its hardness and chemical resistance.

Geological facts

The name 'chalcedony' comes from the ancient Greek town of Chalcedon. It is often found as replacement material in fossils, such as petrified wood. Its microcrystalline nature means its individual crystals are too small to be seen without a microscope.

Field identification & locations

Identify in the field by its waxy luster, smoothness, and 'shell-like' conchoidal fracture edges when broken. It is commonly found in desert environments, riverbeds, and areas with ancient volcanic activity like the American Southwest, Brazil, or India.

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