mineral

Chalcedony (gray-white agate)

Microcrystalline quartz, SiO₂

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; translucent white, gray, and pale tan with a waxy to vitreous luster. Fibrous cryptocrystalline quartz, generally without cleavage, with conchoidal fracture; specific gravity about 2.6.

Identified August 10, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities and fractures in volcanic rocks, precipitating in microscopic quartz fibers; commonly associated with Cenozoic volcanic terrains, though individual specimens may be much younger.

Uses & applications

Used for cabochons, beads, carvings, lapidary slabs, and ornamental objects; durable varieties are also used as decorative stone. Common pieces typically have modest collector value.

Geological facts

The cloudy, layered appearance and waxy surface are characteristic of chalcedony; more distinctly banded material is called agate, while drusy cavities may contain quartz crystals.

Field identification & locations

Identify it by its translucent edges, smooth waxy luster, hardness, and conchoidal fracture; it should scratch glass but not be scratched by a steel knife. Common localities include Brazil, Uruguay, Mexico, India, Namibia, and the western United States.