mineral

Tumbled chalcedony (likely milky agate)

Microcrystalline quartz, SiO₂; chalcedony/agate variety

AI-generated identification · may be incorrect

Translucent pale gray-blue to cream with cloudy yellow-brown inclusions and a waxy-to-vitreous luster; likely Mohs hardness 6.5–7, no visible cleavage, conchoidal fracture, and cryptocrystalline quartz structure. Its smoothly rounded shape indicates tumbling or polishing.

Identified August 18, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities and fractures in volcanic rocks, depositing microcrystalline quartz over time; associated host rocks may range from Paleozoic to Cenozoic in age. Exact locality and age cannot be determined from the image.

Uses & applications

Used for ornamental stones, cabochons, beads, carvings, and inexpensive jewelry; polished specimens are also collected for appearance. It has limited industrial value compared with bulk quartz.

Geological facts

Yellow-brown patches may be iron-oxide staining or included quartz; the blue-gray body is typical of common chalcedony. Color and translucency alone cannot reliably distinguish agate from other chalcedony.

Field identification & locations

Common in volcanic terrains and gravel deposits worldwide, especially Brazil, Uruguay, Mexico, India, Madagascar, and the western United States. Verify with a hardness test against glass and inspect for concentric banding; avoid testing valuable polished pieces.