mineral

Tumbled chalcedony (likely gray agate)

Microcrystalline quartz, SiO₂ (chalcedony; possibly agate)

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; translucent gray to brown with a waxy-to-glassy luster and cryptocrystalline structure. It has no visible cleavage, conchoidal fracture, and moderate durability; the brown spots may be iron-oxide inclusions or surface staining.

Identified September 13, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rock, depositing microscopic quartz fibers and sometimes banding. Most specimens formed in relatively young volcanic terrains, commonly during the Cenozoic, though agate occurs across many geological ages.

Uses & applications

Used for beads, cabochons, carvings, ornaments, and lapidary practice; lower-grade pieces are inexpensive decorative stone.

Geological facts

The rounded shape and smooth surface indicate tumbling or natural abrasion, while the chipped edge exposes the translucent interior. Confirmatory tests include a scratch test against ordinary glass and inspection for conchoidal fracture.

Field identification & locations

Common sources include Brazil, Uruguay, India, Madagascar, Mexico, and the western United States. Agate typically shows banding or moss-like inclusions; plain gray chalcedony may lack obvious bands.