mineral

Tumbled chalcedony (likely blue-gray agate)

Cryptocrystalline quartz, SiO₂; chalcedony/agate variety

AI-generated identification · may be incorrect

Pale blue-gray, translucent to semi-opaque, with a waxy-to-glassy luster and rounded tumbled surface; Mohs hardness 6.5–7, no visible cleavage, trigonal quartz structure, specific gravity about 2.60–2.65.

Identified August 9, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities and fractures in volcanic rocks, often developing microscopic quartz fibers and subtle banding. The specimen is likely geologically ancient, commonly associated with Cenozoic to older volcanic terrains.

Uses & applications

Used for cabochons, beads, carvings, lapidary work, and inexpensive collector specimens; polished agate is also decorative.

Geological facts

Chalcedony’s cryptocrystalline texture makes it tougher than many visibly crystalline quartz specimens. Blue-gray coloration may reflect inclusions, microscopic structure, or mild enhancement.

Field identification & locations

Identify by its smooth waxy appearance, hardness, translucency, and lack of obvious crystal faces; it should resist scratching by a steel knife but scratch glass. Common sources include Brazil, Uruguay, India, Mexico, and the western United States.